Multi-beam charged-particle microscope design using adaptive detection system

Abstract

To provide a multi-beam charged-particle beam system that comprises a detection unit configured for dynamic compensation of a charge effect of a sample.SOLUTION: A detection unit comprises a relay optical system that includes an adaptive mirror array for maintaining a signal beam in a position of an inlet aperture of an optical waveguide or a detection element. Thereby, beam distortion induced by a charge effect is compensated for. The present invention can be applied to a wafer inspection using a multi-beam charged-particle beam system.SELECTED DRAWING: Figure 5a

Description

[Technical Field]

[0001] The present disclosure relates to a multi-beam charged particle microscope with reduced and improved imaging contrast and a method for inspection of semiconductor features with improved image contrast. [Background technology]

[0002] WO 2005 / 024881 discloses an electron microscope system operating with multiple electron beamlets to parallel scan an object to be inspected with the bundle of electron beamlets. The bundle of primary charged particle beamlets is generated by guiding a primary charged particle beam onto a multi-beam forming unit including at least one multi-aperture plate having a plurality of apertures. A portion of the electrons of the electron beam impinge on the multi-aperture plate and are absorbed there, while another portion of the beam transmits through the apertures of the multi-aperture plate, thereby forming electron beamlets having a cross section defined by the cross section of the aperture in the beam path downstream of each aperture. The primary charged particle beamlets are focused onto the surface of the sample by an objective lens, causing secondary electrons or backscattered electrons to emanate from the sample as secondary electron beamlets, which are collected and imaged onto a detector. Each secondary beamlet impinges on a separate detector element or group of detector elements, so that the detected secondary electron intensity provides information about the surface of the sample at the location where the corresponding primary beamlet impinges on the sample. The bundle of primary beamlets is systematically scanned over the surface of the sample and an electron microscope image of the sample is produced in the usual manner for a scanning electron microscope.

[0003] Generally, the imaging contrast of a scanning electron microscope depends on the signal generated by secondary electrons, which depends on the secondary electron (SE) yield per primary electron and the geometric collection efficiency of the electron microscope. The SE yield depends on the material properties and the kinetic energy of the primary electrons. The secondary electron beamlets are then focused by the objective lens and directed to a detector. However, secondary electrons generated and extracted from the sample surface are often susceptible to charging effects at the sample surface, especially when the secondary electron yield is not balanced with the incident primary electron current. These charging effects lead to degradation of the secondary electron beamlets and reduced image contrast, increased crosstalk, or even complete loss of the secondary electron signal. Charging effects further degrade image contrast in wafer inspection tasks during integrated circuit manufacturing. Such wafers contain semiconductor materials, local capacitances, and insulators, which can accumulate surface charges, for example. In another example, the target of an inspection task is a wafer covered with photoresist, which accumulates local surface charges. Patent application WO 2022 / 248141 and German patent application No. 102022114923.4 disclose monitoring methods for detecting charging effects in such charged samples. German patent application No. 102018124044B3 proposes deconvolution of crosstalk. However, deconvolution is only feasible for minor charging effects. Generally, electron-optical measures are required to maintain high contrast in the secondary electron detector in order to compensate for charging effects during imaging of secondary electron beamlets.

[0004] Different mechanisms have been proposed to improve the imaging contrast of multi-beam electron microscopes in the presence of charging effects. U.S. Pat. Nos. 1,104,968,668, U.S. Pat. Nos. 1,089,680, 1,081,121,5 B2, and WO 2021,239,380 propose the placement of several active electrostatic or magnetically driven elements in the secondary electron imaging system. However, these systems in the art are either highly complex or do not allow for fast correction of degraded secondary electron beamlets with sufficient correction magnitude. For example, U.S. Pat. No. 10,811,215 B2 proposes a highly complex secondary electron imaging system design that includes up to nine electron-optical lenses.

[0005] However, many electron-optical solutions proposed for compensation of charging effects translate into very complex electron-optical systems. Therefore, there is a further need to provide secondary electron imaging systems that offer simpler means of compensation for charging effects. Furthermore, the increasing demand for higher throughput requires improvements in the collection efficiency of secondary electron signals. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2005 / 024881 [Patent Document 2] International Publication No. 2022 / 248141 [Patent Document 3] German Patent Application No. 102022114923.4 [Patent Document 4] German Patent Application No. 102018124044B3 [Patent Document 5] U.S. Patent No. 11049686BB [Patent Document 6] U.S. Patent No. 10896800BB [Patent Document 7] U.S. Patent No. 10811215B2 [Patent Document 8] International Publication No. 2021239380 Summary of the Invention

[0007] The present invention provides a multi-beam charged particle beam system and a method of operating the multi-beam charged particle beam system for image acquisition with higher imaging contrast and increased throughput, even in the presence of charging effects. The objects of the present invention are achieved by an improved imaging system design.

[0008] This patent application claims priority from German Patent Application No. 102024205080.6, filed on June 3, 2024, the disclosure of which is incorporated in its entirety into this patent application by reference.

[0009] In a first embodiment, a method of operating a multi-beam charged particle beam system is provided, the method comprising: - forming a plurality of first focused spots of a plurality of primary charged particle beamlets at a first image plane; - focusing the plurality of secondary electron beamlets to form a plurality of second focused spots of the plurality of secondary electron beamlets at a second image plane; - converting the plurality of second focused spots into a plurality of light beams to form the plurality of third focused spots at a third image plane; - individually adjusting the position of the at least one third focused spot; - receiving a plurality of intensity signals using a plurality of high speed detection elements; Contains:

[0010] In one example, steps are performed in parallel during image acquisition, thereby compensating for distortions of the secondary electron beamlets during imaging due to, for example, charging effects on the surface of the object.

[0011] In one example, the method further includes monitoring multiple positions of the multiple third focused spots with a high-resolution image sensor and determining a deviation of the position of at least one third focused spot from a predetermined position. For example, the method further includes determining from the deviation a tilt angle of at least one mirror of the adaptive mirror array, and individually adjusting the position of the at least one third focused spot by adjusting the tilt angle of at least one mirror of the adaptive mirror array.

[0012] In one example, the method further includes adjusting the position or rotation angle of a plurality of entrance apertures of an optical waveguide connected to the high-speed detection element using a translation stage.

[0013] According to a second embodiment, a multi-beam charged particle beam system includes an objective radiation unit configured for irradiating a surface of an object located at a first image plane with a plurality of first focused spots of a plurality of primary charged particle beamlets. The multi-beam charged particle beam system further includes a detection unit configured to collect a plurality of secondary electron beamlets excited at the plurality of first focused spots from the surface of the object and form a plurality of second focused spots of the plurality of secondary electron beamlets at a second image plane of the detection unit. The multi-beam charged particle beam system further includes an electron-to-photon conversion unit located at the second image plane of the detection unit, and a relay optical system for imaging photons or light generated by the electron-to-photon conversion unit and for forming a plurality of third focused spots at a third image plane of the relay optical system. A plurality of entrance apertures of optical waveguides are located at the third image plane. Each of the optical waveguides is connected to a high-speed detection element. In one example, the high-speed detection elements are located directly in the third image plane.

[0014] The relay optical system further includes an adaptive mirror array and a mirror control module configured to control the tilt angles of a plurality of mirrors of the adaptive mirror array such that the positions of a plurality of third focusing points are maintained constant at a plurality of entrance apertures of an optical waveguide or a high-speed detection element during image acquisition.

[0015] In one example, the multi-beam charged particle beam system further includes a monitoring system including a high-resolution sensor. The high-resolution sensor is arranged to receive an image of the positions of the plurality of third focusing points. For example, the relay optical system includes a beam divider mirror for splitting light and guiding the split light of a plurality of light rays onto the high-resolution sensor. The mirror control module is configured to determine a position deviation from a plurality of entrance apertures of an optical waveguide or a high-speed detection element at the plurality of third focusing points. In one example, the mirror control module is further configured to determine the tilt angle of the adaptive mirror array from the position deviation.

[0016] In one example, the relay optical system has a magnification of M>=20, for example, M = 3, M = 40, or more, thereby enabling separation of a plurality of light rays and individual control of each position of the plurality of third focusing points. In one example, the adaptive mirror array is arranged at a distance L1 to the third image plane, where L1>=JX×P1 / 2, JX is the number of a plurality of beamlets in one lateral direction, and P1 is the pitch of the third focusing spot. In one example, L1>16.5 mm, for example, L1>45 mm, or, for example, L1>80 mm. However, the distance L1 must be determined according to JX beamlets in one lateral direction and must not exceed the maximum distance L1max, where L1max is given by L1<L1max = P1×[1 / NA1 - JX] / 2, and NA is the numerical aperture of the light rays at the third image plane. In one example, L1>20 mm.

[0017] In one example, each of the multiple mirrors is an individually and continuously tiltable mirror having at least one flexure and at least one actuator. In one example, the multiple entrance apertures of the optical waveguide or the high-speed detection element are mounted on a motion stage for adjusting the lateral position or rotation angle of the multiple entrance apertures of the optical waveguide or the high-speed detection element relative to the multiple third focused spots. This allows for adjusting the wide-area rotation angle or wide-area lateral displacement of the multiple third focused spots. The relay optical system may further include a zoom lens for adjusting the magnification of the relay optical system.

[0018] According to one embodiment, the multi-beam charged particle beam system comprises a control unit comprising software code for causing the multi-beam charged particle beam system to perform the method according to the first embodiment. In one example, the mirror control module is an ASIC programmed to perform the method steps of determining a deviation of a position of the at least one third focal spot from a predetermined position, determining a tilt angle of at least one mirror of the adaptive mirror array from the deviation, and individually providing control signals to the adaptive mirror array for adjusting the position of the at least one third focal spot by adjusting the tilt angle of at least one mirror of the adaptive mirror array.

[0019]

[0013] Embodiments or examples of the present invention provide a multi-beam charged particle beam system and a method of operating a multi-beam charged particle beam system with improved image contrast and improved image signal. Thus, the present invention enables wafer inspection, including charged wafer specimens, with higher precision and accuracy. It will be understood that the present invention is not limited to the embodiments and examples, but also includes combinations and variations of the embodiments and examples.

[0020] Embodiments of the present disclosure will now be described in more detail with reference to the drawings. [Brief explanation of the drawings]

[0021] [Figure 1] 1 is a schematic cross-sectional view of a multi-beam charged particle beam system 1. FIG. [Figure 2] FIG. 1 shows some details of a multi-beam charged particle beam system 1. [Figure 3] FIG. 1 shows some further details of a multi-beam charged particle beam system 1. [Figure 4] FIG. 6 shows a detector 600 including an optical relay system. [Figure 5a] FIG. 1 illustrates an optical relay system according to one embodiment. [Figure 5b] FIG. 1 illustrates an optical relay system according to one embodiment. [Figure 5c] FIG. 1 illustrates an optical relay system according to one embodiment. [Figure 5d] FIG. 1 illustrates an optical relay system according to one embodiment. [Figure 6a] FIG. 1 illustrates some design constraints of an optical relay system according to one embodiment. [Figure 6b] FIG. 1 illustrates some design constraints of an optical relay system according to one embodiment. [Figure 7] FIG. 1 illustrates a method of operation according to one embodiment. [Figure 8a] 1 is an illustration of an example of the effect of the method. [Figure 8b] 1 is an illustration of an example of the effect of the method. [Figure 8c] 1 is an illustration of an example of the effect of the method. DETAILED DESCRIPTION OF THE INVENTION

[0022] In the exemplary embodiments of the present invention described below, functionally and structurally similar components are designated, whenever possible, by similar or identical reference numerals.

[0023] Some array elements, e.g., multiple primary charged particle beamlets, are identified by reference numbers. Depending on the context, the same reference number may also identify a single element from multiple elements. Each primary charged particle beamlet (3.1, 3.2, 3.3) is one beamlet among the multiple primary charged particle beamlets (3).

[0024] The schematic depiction in FIG. 1 illustrates the basic features and functionality of a multi-beam charged particle system 1. Note that the reference numerals used in the figure are chosen to symbolize their respective functionality. The type of system shown is a multi-beam scanning electron microscope using multiple primary charged particle beamlets 3 to generate multiple primary charged particle beam spots 5 on a surface 25 of an object 7, such as a wafer or mask substrate, positioned with its top surface 25 in the object plane 101 of an objective lens 102. For simplicity, only three primary charged particle beamlets 3.1-3.3 and three primary charged particle beam spots 5.1-5.3 are shown. The features and functionality of the multi-beamlet charged particle system 1 can be implemented using electrons or other types of primary charged particles, such as ions, particularly helium ions. Further details of the microscope system 1 are provided in International Patent Application WO 2022262970, filed June 16, 2021, which is incorporated herein by reference in its entirety.

[0025] The system 1 comprises an objective radiation unit 100, a detection unit 200 and a secondary electron beam divider or beam splitter unit 400 for separating a secondary charged particle beam path 13 from a primary charged particle beam path. The objective radiation unit 100 comprises a charged particle multi-beam generator 300 for generating a plurality of primary charged particle beamlets 3 and is adapted to focus the plurality of primary charged particle beamlets 3 onto an object plane 101, at which a surface 25 of an object or wafer 7 is positioned by a sample stage 500.

[0026] The primary beam generator 300 produces a plurality of primary charged particle beamlet spots at an intermediate image plane 321. The primary beamlet generator 300 comprises at least one source 301 of primary charged particles, e.g., electrons. The at least one primary charged particle source 301 emits a diverging primary charged particle beam, which is collimated by at least one collimating lens 303 to form a collimated or parallel primary charged particle beam 309. The collimating lens 303 typically consists of one or more electrostatic or magnetic lenses, or a combination of electrostatic and magnetic lenses. The collimated primary charged particle beam 309 enters a primary multi-beam forming unit 305. The multi-beam generating unit 305 is described, for example, in U.S. Patent Application Publication No. 2019 / 0259575 and U.S. Patent No. 10,741,355 B1, both of which are incorporated herein by reference. The multi-beam forming unit 305 essentially comprises a first multi-aperture plate or filter plate 304 illuminated by a collimated primary charged particle beam 309. The first multi-aperture plate or filter plate 304 comprises a plurality of apertures in a raster configuration for generation of a plurality of primary charged particle beamlets 3, which are generated by transmission of the collimated primary charged particle beam 309 through the plurality of apertures. The multi-beamlet forming unit 305 comprises at least one further multi-aperture plate 306 arranged downstream from the first multi-aperture or filter plate 304 with respect to the direction of motion of electrons of the beam 309. The multi-beamlet forming unit 305 is further configured with an adjacent electrostatic field lens 331 coupled within the multi-beamlet forming unit 305 in some examples. In combination with the second field lens 333, the multiple primary charged particle beamlets 3 are focused at or close to the intermediate image plane 321. The primary charged particle source 301 and each of the active multi-aperture plates 306 are controlled by a control unit 830.

[0027] The multiple focal points of the primary charged particle beamlets 3 formed near the intermediate image plane 321 are imaged by the field lens group 103 and the objective lens 102 onto the object plane 101, where the surface 25 of the object 7 is positioned. A decelerating electrostatic field is generated between the objective lens 102 and the object surface 25 by application of a voltage to the object by the specimen voltage source 503. The decelerating electrostatic field generated by the specimen voltage source 503 adjusts the landing energy EL of the primary electrons to, for example, less than 2 keV, less than 1 keV, less than 800 eV, less than 500 eV, less than 300 eV, or even lower.

[0028] FIG. 2 shows further details of the generated decelerating electrostatic field. A multi-aperture structure 305 generates multiple primary charged particle beamlets 3 from a collimated electron beam 309. Again, for simplicity, only three beamlets 3.1-3.3 are shown in FIG. 2, but there may be many more beamlets, e.g., more than 60, more than 90, or even more than 300. A beam tube 151 is provided downstream from the multi-aperture structure 305 and is connected to a voltage source having a first voltage, or tube voltage, VT. The multiple primary charged particle beamlets 3 are at a constant kinetic energy ET from the entrance of the beam tube 151 to the exit aperture 153 of the beam tube 151. The kinetic energy ET of the primary charged particle beamlets 3 during their passage through the beam tube 151 may be, for example, 20 keV, 30 keV, or greater.

[0029] The multiple primary charged particle beamlets 3 are imaged to focus points 5.1-5.3 at the object plane 101 by the field lenses 333 and 103 and by the objective lens 102. The objective lens 102 is a magnetic lens type comprising a coil 161 and a pole piece 163 including a lower pole piece segment 165, forming an axial gap for the magnetic field. Other types of magnetic lenses are also feasible, such as a radial gap lens for generating an immersion lens field, or a magnetic lens comprising several coils and pole pieces. A beam divider 400 is arranged upstream of the objective lens 102 or partially integrated into the objective lens 102, and is configured to separate secondary electrons along the secondary electron beam path to the detector unit 200. An electrode 133 is provided below the lower pole piece segment 165 and is connected to a voltage source for supplying a second voltage VE to the electrode. In the example shown, electrode 133 is provided as a separate electrode.

[0030] A specimen voltage VL is supplied by the specimen voltage source 503 to a specimen mounting platform 505 for holding and contacting the wafer 7 during use. At the surface 25 of the wafer 7, a first material composition 67 is disposed directly beneath the first set of primary charged particle beamlets 3.1 and 3.2, and a second material composition 69 is disposed directly beneath the second set of primary charged particle beamlets including the primary charged particle beamlet 3.3. An electric field 137 is generated according to the voltage difference between VL and VT, which is approximately parallel to the propagation direction of the primary charged particle beamlets 3 and generates a deceleration force on the primary charged particles (FIG. 2 shows the equipotential lines of the electric field 137). The specimen voltage VL is adjusted so that the third kinetic energy or landing energy EL of the primary electrons is within a range of less than 5 keV, less than 2 keV, less than 800 eV, less than 300 eV, or even less than 100 eV. The electric field 137 forms a deceleration field for reducing the kinetic energy of the primary charged particle beamlets 3 before they impinge on the sample surface 25 arranged in the object plane 101 so as to achieve high resolution. The electric field 137 further forms an extraction field for extracting and accelerating secondary electrons from the wafer 7. Therefore, the electric field 137 is also referred to as the extraction field 137.

[0031] The objective radiation system 100 of the multi-beam charged particle beam system 1 shown in Figures 1 and 2 further comprises a collective multi-beam raster scanner 110 adjacent to the beam crossover 108, by which the multiple charged particle beamlets 3 can be deflected in a scanning direction perpendicular to the propagation direction of the charged particle beamlets. The propagation direction of the primary beamlets throughout the example is the positive z direction. The objective lens 102 and the collective multi-beam raster scanner 110 are adjusted to center an optical axis (not shown) of the multi-beam charged particle system 1, which is perpendicular to the wafer surface 25. The multiple primary charged particle beamlets 3, which form multiple beam spots 5 arranged in a raster configuration, are scanned synchronously over the wafer surface 25. In one example, the raster configuration of the focused spots 5 of the plurality of J primary charged particle beams 3 is a hexagonal raster of approximately 100 or more primary charged particle beams 3, e.g., J=91, J=100, or J=approximately 300 or more beams. The primary beam spots 5 have a distance of approximately 6 μm to 45 μm and a diameter of less than 5 nm, e.g., 3 nm, 2 nm, or even smaller. In one example, the beam spot size is approximately 3 nm, and the distance between two adjacent beam spots is 8 μm. Multiple secondary electrons are generated at each scanning position of each primary beam spot of the plurality of primary beam spots 5, each forming multiple secondary electron beamlets in the same raster configuration as the primary beam spot 5. The intensity of the secondary charged particle beamlets generated at each beam spot 5 depends on the intensity of the impinging primary charged particle beams 3, the material composition 67, 69 and topography of the object 7 directly below the beam spot 5, and the charge state of the sample at the beam spot 5. The multiple secondary charged particle beamlets are accelerated by the same electrostatic field 137 between the objective lens 102 and the object surface 25, are focused by the objective lens 102, and pass through the first collective multi-beam raster scanner 110 in the opposite direction to the primary beamlets 3. The multiple secondary beamlets are deflected by the first collective multi-beam raster scanner 110 during scanning.The multiple secondary charged particle beamlets are then guided by a secondary electron beam divider or beam splitter unit 400 to follow a secondary beam path 13 to the detection unit 200. The beam splitter unit 400 is typically configured to separate the secondary beam path 11 from the primary beam path by means of a magnetic field or a combination of magnetic and electrostatic fields.

[0032] The detection unit 200 images the secondary electron beamlets onto the image sensor 600 to form a plurality of secondary charged particle image spots 15 thereon. FIG. 3 shows an example of the detection unit 200 and further components already shown in FIG. 1 or 2, which are labeled with the same reference numerals. Reference is made to the description of FIGS. 1 and 2. The primary charged particle beamlets are indicated diagrammatically by the primary beam path 11. FIG. 3 shows the secondary electron beam paths of two exemplary secondary electron beamlets 9.i and 9.o. There are a number of further secondary electron beamlets corresponding to the number of primary charged particle beamlets focused on the surface 25 of the sample 7 (only the focusing points 5.o and 5.i are shown). The detection unit 200 comprises a second branch 151.2 of the common beam tube 151 connected to a voltage supply line and set to a tube voltage VT (see also FIG. 2). VT may be, for example, ground potential. Through the tube 151, the primary charged particles propagate with a constant, high kinetic energy, for example, E1 = 30 keV. The detection unit 200 further includes a second beam tube segment 155 with a tube voltage VT3. In one example, VT and VT3 are identical and are all set to ground level. A first magnetic projection lens 205.1 and a second scanning deflector 222 are disposed within the detection unit 200. The second raster scanner 222 is connected to a scanning control unit 860. The scanning control unit 860 is configured to compensate for differences in scanning deflection power of the first scanning deflector 110 within the common beam path so that the positions of the multiple secondary electron focal spots 15 remain constant at the image sensor 600. In this example, the second scanning deflector 222 is a two-stage electrostatic octupole scanner disposed within the second branch 151.2 of the common beam tube 151. Within the detection unit 200 there is provided at least one electrostatic deflector or multipole corrector 220 for quasi-electrostatic adjustment of the secondary electron beam path and two further magnetic projection lenses 205.2 and 205.3.A pair of two further magnetic projection lenses 205.2 and 205.3 are configured to form focused spots 15.i, 15.o of the secondary electron beamlets 9.i, 9.o on the image plane 225 and to adjust image rotation of the secondary electron beamlets caused, for example, by a change in the object plane 101 by the objective lens 102. The three magnetic projection lenses 205.1, 205.2, and 205.3 and the quasi-electrostatic multipole corrector 220 are connected to and controlled by the secondary beam path control module 840. Additional magnetically driven multipole deflectors (not shown) may be arranged for quasi-electrostatic adjustment of the beam paths 13 of the multiple secondary electron beamlets 9.

[0033] System 1 may further optionally comprise a retractable monitoring system 230 (see FIG. 1). Such monitoring systems and methods for detecting charging effects in charged samples are further described in patent applications WO 2022248141 and DE 102022114923 A1, which are incorporated herein by reference in their entirety. Detection unit 200 is described in more detail below.

[0034] The stage 500 preferably does not move during acquisition of an image section by scanning the multiple primary charged particle beamlets 3, and after acquisition of the image section the stage 500 moves to the next image section to be acquired. In an alternative embodiment, with the collective multi-beam raster scanner 110 in a first direction, the stage 500 is continuously moved in a second direction while an image is acquired by scanning the multiple primary charged particle beamlets 3. The stage movements and stage positions are monitored and controlled by sensors known in the art, such as laser interferometers, grating interferometers, confocal microlens arrays, or the like.

[0035] During image scanning, the control unit 800 is configured to trigger the image sensor 600 to detect multiple timely resolved intensity signals from the multiple secondary electron beamlets at predetermined time intervals, and digital images of the image section are accumulated and stitched together from all scanning positions of the multiple primary charged particle beamlets 3.

[0036] The control unit 800 of the multi-beamlet charged particle system 1 further comprises an imaging control module 810 configured to receive a data stream from the image sensor 600 and to generate a digital image of the surface of the sample 7 during operation, a secondary beam path control module 840 configured to control the detection unit 200, a primary beam path control module 830 configured to control elements of the objective radiation unit 100, a stage control module 850 configured to control stage positioning and alignment, including control of the sample voltage supply unit 503, a scanning operation control module 860 configured to control scanning operations by the first collective multi-beam raster scanner 110 and the second deflection system 222, a control operation processor unit 880 configured to perform inspection tasks of the sample and to control the modules 810, 820, 830, 840, 850, 860, and a memory 890 for storing software, instructions, and image data. The control operation processor unit 880 is further connected to an interface IX for exchanging data, instructions, software or user interaction.

[0037] The image sensor 600 is configured with an array of sensing areas in a pattern that matches the raster arrangement of secondary electron beamlets focused onto the image sensor 600 by the detection unit 200. This allows detection of each individual secondary electron beamlet independently of other secondary electron beamlets incident on the image sensor 600. According to an embodiment of the present invention, the image sensor 600 is configured as an electron-to-photon conversion unit or scintillator plate 602 located at the focal plane 225 of the multiple secondary electron particle image spots 15. In this embodiment shown in FIG. 4 , the image sensor 600 comprises a relay optical system 603 including a condenser lens 605 and a zoom lens 611 for imaging and directing photons generated by the electron-to-photon conversion unit 602 at the secondary charged particle image spots 15 to an image plane 613. Dedicated photon detection elements 623, such as multiple photomultiplier tubes or avalanche photodiodes, may be located at the image plane 613. The photons generated by the electron-to-photon conversion unit 602 form multiple light beams 609, of which only one light beam 609.3, corresponding to the secondary electron focal point 15.3, is shown in FIG. 4 . Using a relay optical system 603, the photon conversion unit 602 is imaged onto an image plane 613 where the entrance aperture of an optical waveguide 615 is located. Each optical waveguide 615 receives an optical intensity corresponding to a respective secondary charged particle image spot 15, e.g., spot 15.1, 15.2, or 15.3, and each optical waveguide 615 is connected to a respective detection element 623. For example, the relay optical system 603 forms a focal point 619.3 of the light beam 609.3 at the image plane 613, and the entrance aperture of the optical waveguide 615.3 is located at the image position of the secondary electron focal spot 15.3. This allows photons generated at each focal point 15 of each secondary electron beamlet 9 to be detected individually. The optical waveguides may be mounted with a regular raster spacing on a translation stage 617, and the rotational lateral positions of the apertures of the optical waveguides 615 may be adjusted by the translation stage 617 according to the raster position and rotation of the secondary electron beamlets 9 in the image plane 225. Figure 4 shows only three focused spots 15.1-15.3 and three corresponding optical waveguides 615 and detector elements 623.However, the number of corresponding optical waveguides 615 and detector elements 623 may be much higher, their number being at least given by the number J of secondary electron beamlets 9 .

[0038] The image sensor 600 is further configured with a beam divider mirror 237, an imaging lens 235, and a monitoring system 230 comprising a high-resolution CMOS sensor 232, whereby the positions of the multiple focused spots 15 on the electron-to-photon conversion unit 602 are monitored during use.

[0039] The intensity is detected individually for each of the multiple secondary charged particle beam spots 15, and characteristics of the object surface 25 are detected with high throughput and high resolution for large image sections of the object 7. For example, with a 10 x 10 beamlet raster having an 8 μm pitch, a single image scan using the collective multi-beam raster scanner 110 generates an image section of approximately 88 μm x 88 μm, for example, with an image resolution of 2 nm or less. The image section is sampled at half the beam spot size, thus 8,000 pixels per image line for each beamlet, so that an image section generated by 100 beamlets has 6.4 gigapixels. Digital image data is collected by the control unit 800. Details of digital image data collection and processing, for example using parallel processing, are described in International Patent Application WO 2020 / 151904 and U.S. Patent No. 9,536,702, which are incorporated herein by reference.

[0040] However, when sample charging occurs, the trajectory of the secondary electron path is distorted. As a result, the detector 600 is no longer accurately hit by the secondary electron beamlets. In the worst case, this can result in a dark image or extensive crosstalk. Due to the distortion, the focal point 15 of the secondary electron beamlets 9 is displaced, and the excited light from the electron-to-photon conversion unit 602 is not imaged onto the corresponding entrance aperture of the optical waveguide 615 or the detection element 623.

[0041] Thus, according to one embodiment, the relay optical system 603 comprises an adaptive system for individually adjusting the focusing points of the rays of the beam bundle 609. An example is shown in Fig. 5. Fig. 5a shows an overview of the relay optical system 603, which comprises a condenser lens 605, a tube or zoom lens 611, a beam divider 237, a monitoring high-resolution sensor 232, an adaptive mirror array 671, and a plurality of detector elements 623 connected to an imaging control module 810. The condenser lens 605 condenses the light generated by the electron-to-photon conversion unit 602, and the tube or zoom lens 611 focuses the bundle of rays 609 onto the detector elements 623 (or, respectively, onto the entrance aperture 613 of the optical waveguide 615). At a distance L1 to the detector elements 623, an array of tiltable mirrors in the form of an adaptive mirror array 671 is arranged. The distance L1 is selected so that each mirror of the mirror array 671 is intersected by one ray, e.g., by each ray 609.1, 609.2 corresponding to one focal point 15 or one individual secondary electron beamlet 9 (see FIGS. 3 and 4). The tilt angle of each tiltable mirror of the adaptive mirror array 671 is adjusted so that each individual ray 609, e.g., ray 609.1, is adjusted to intersect with a corresponding detector element 623.1. The tilt angle is determined by the mirror control module 820 in response to the distorted positions of the beamlets 609.1b-609.3b. The distorted positions of the beamlets 609.1b-609.3b are detected by monitoring the high-resolution sensor 232. Thereby, during operation, multiple secondary electron beamlets 9 are excited at the surface 25 of the object 7 by radiation of the multiple primary electron beamlets 3 at the surface 25 of the object 7, which includes the focal points 5. The multiple secondary electron beamlets 9 are focused onto the electron-photon conversion unit 602 by the detection unit 200, and multiple light rays 609 are excited by the electron-photon conversion unit 602. Each light ray of the multiple light rays 609 is a corresponding one of the multiple secondary electron beamlets 9, and each corresponding one of the multiple primary electron beamlets 3, respectively.Each position of the image point of each ray of the plurality of light rays 609 is detected by monitoring the high-resolution sensor 232. The mirror control module 820 is configured to determine a distortion of the image point of each ray of the plurality of light rays 609 after passing through the adaptive mirror array 671 and trigger an adjustment of at least one tilt angle of the adaptive mirror array 671, so that the image point of each ray of the plurality of light rays 609 after passing through the adaptive mirror array 671 hits a corresponding entrance aperture of the light guide 615 or the detector element 623, respectively. The relay optical system 603 is folded at the adaptive mirror array 671 at a folding angle defined by the angle between the normal to the adaptive mirror array 671 and the symmetry axis 608 of the collecting lens 605.

[0042] 5b shows an example of a collecting lens 605. The collecting lens 605 may be similar to a microscope lens, with a collecting aperture having an NA of 0.6 or greater, e.g., NA=0.8. The collecting lens 605 may comprise a series of lens elements including, for example, 10 or more lens elements consisting of singlets, doublets, or triplet compound lenses. The lens elements are centered along an optical axis 608.

[0043] FIG. 5c shows a simplified example of an adaptive mirror array 671 including three tilting mirrors 673.1, 673.2, and 673.3. Each tilting mirror 673.1-673.3 is attached to the mirror body of the adaptive mirror array 671 via hinges 677.1-677.3. Multiple electrode segments 675a-675b are positioned below each mirror 673. This allows the tilt of each mirror 673 to be individually adjusted by electrostatic force. In addition to the multiple electrode segments 675a-675b functioning as actuators, capacitive sensors may be provided to directly monitor the tilt angle of each mirror. Other examples of continuously adjustable tilt angles of mirror arrays are similarly feasible, involving articulations and actuators, including, for example, piezo actuators, and optionally sensors for active feedback control of the tilt position of each mirror.

[0044] Figure 5d shows an example where light ray 609.1a is distorted and does not intersect at the center of detection unit 623.1. The displacement of light ray 609.1a is detected by monitoring high-resolution sensor 232 (see Figure 5a), and a corresponding signal for adjusting the focal position of light ray 609.1a is determined by mirror control module 820 and provided to actuators 675.1a, 675.1b of tilting mirror 677.1. Thereby, tilt angle 679.1 is adjusted so that light ray 609.1a is deflected according to beam path 609.1b and appropriately adjusted to intersect at the center of detection unit 623.1.

[0045] The optical properties of the relay optical system 603 are further illustrated by Figure 6. The system magnification M of the relay optical system 603 is limited by the following properties: the pitch P0 of the focusing points 15 that form the points of origin of the light rays 609; the pitch of the beamlets at the detector elements 623 or entrance aperture 613 of the optical waveguide 615 is P1=M*PO. Collection NA0 (numerical aperture, half the sine of the collection opening angle), for example NA0>0.4, for example NA0>=0.6, or for example NA0=0.8. The image-side numerical aperture at the detector element 623 or entrance aperture 613 of the light guide 615 is NA1=NA0 / M. the number JX of unidirectional beamlets forming the diameter of the beamlet in the X direction, D1=JX×P1.

[0046] The distance L1 between the adaptive mirror array 671 and the image plane, where for example the detection unit 623 is located, must be chosen so that the individual beamlets do not overlap. For a system folding angle of 45°, the following condition should be obtained: (1) L1 <L1max=P1×[1 / NA1-JX] / 2。 On the other hand, the distance L1 between the adaptive mirror array 671 and the image plane 613 is (2) L1>L1min=JX×P1 / 2 It should exceed that. Since the relay optical system 603 with adaptive mirror array 671 must tolerate some distortion of the beamlets to be compensated for, the allowable system solution space 691 has some spacing relative to conditions (1) and (2) given above.

[0047] In a typical example, the number of beamlets JX is 9 in the radial direction, and the total number of beamlets in the hexagonal arrangement of beamlets is J=61. The magnification of the relay optical system 603 is selected to be greater than M=20. The distance L1 between the adaptive mirror array 671 and the detection unit 623 or the entrance aperture of the optical waveguide, respectively, is 17 mm to 26 mm. The total system length of such a relay optical system 603 is about 500 mm.

[0048] In a typical example, the number of beamlets JX is 17, and the total number of beamlets J=217. In this example, the magnification of the relay optical system 603 is selected to be greater than M=31. The distance L1 between the adaptive mirror array 671 and the detection unit 623 or the entrance aperture of the optical waveguide, respectively, is 46 mm to 57 mm. The total system length of such a relay optical system 603 is about 770 mm.

[0049] 6b shows the design requirements of the relay optical system 603 for different numbers of beamlets JX in the radial direction, where the pitch P1 is about 3 mm to 4 mm.

[0050] FIG. 7 illustrates a method of operation of the multi-beam charged particle beam system 1.

[0051] A method for operating a multi-beam charged particle beam includes step S1 including forming a plurality of first focused spots of a plurality of primary charged particle beamlets at a first image plane, focusing a plurality of secondary electron beamlets, and forming a plurality of second focused spots of the plurality of secondary electron beamlets at a second image plane, and step S1 further includes converting the plurality of second focused spots into a plurality of light beams and forming a plurality of third focused spots of the plurality of secondary electron beamlets at a third image plane.

[0052] In step S1, image acquisition of the surface 25 of a substrate, e.g., a wafer 7, is initiated. A plurality of secondary electron beamlets 9 are excited from the surface 25 of the object 7. The plurality of secondary electron beamlets 9 are imaged onto the electron-to-photon conversion unit 602, and each beamlet is individually converted into a light beam having an intensity corresponding to the secondary electron yield at the corresponding emission position of the object 7. Each signal of each secondary electron beamlet 9 is individually acquired by the detection element 623. A small portion is separated by the optical beam divider mirror 237, and the focal point of the plurality of light beams 609 is monitored by the high-resolution sensor 232.

[0053] The method further includes step S2 of monitoring multiple positions of the multiple third focused spots using a high-resolution image sensor and determining a deviation of the position of at least one third focused spot from a predetermined position. Step S2 further includes determining a tilt angle of at least one mirror of the adaptive mirror array from the deviation. In step S2, the focused points of the multiple light beams 609 obtained by the high-resolution sensor 232 are determined by image processing and compared to target positions of the focused points of the multiple light beams 609. Such target positions may be determined in advance during system calibration and stored in the memory of the mirror control module 820. Image processing methods are described, for example, in PCT / EP2024 / 051248, filed January 19, 2024, which is incorporated herein by reference.

[0054] A distortion of each individual beamlet is determined from a comparison of the actual position and the target position, and at least two tilt angles of the corresponding mirror element 673 are determined. The change in tilt angle is determined, for example, according to a predetermined sensitivity of tilt angle dA per displacement D for each individual mirror 673 for each corresponding beamlet 609.

[0055] The method for operating a multi-beam charged particle beam further includes step S3 of individually adjusting a position of at least one third focused spot and receiving a plurality of intensity signals using a plurality of high-speed detection elements (623). In step S3, the tilt angle determined in step S2 is applied to each mirror element 673 of the adaptive mirror array 671. Thereby, the position of the at least one third focused spot is adjusted by actuation of a corresponding mirror of the adaptive mirror array. Thereby, the position of each third focused spot is individually adjusted by actuation of each corresponding mirror of the adaptive mirror array.

[0056] In one example, steps S2 and S3 operate continuously during image acquisition, thereby compensating for varying charging effects individually for multiple beamlets, thereby compensating for distortions of secondary electron beamlets during imaging due to, for example, charging effects on the surface of the object.

[0057] In one example, the method further includes using a translation stage to adjust the position or rotation angle of a plurality of entrance apertures of an optical waveguide connected to the high-speed detection element.

[0058] FIG. 8 shows an example of adaptive correction of the position of the focal point 619. FIG. 8a shows an ideal situation of the beam spot 619 at the image plane 613 of the relay optical system 613, without charging effects. In the example, J=9 and the number of beamlets J=61, although there may be more beamlets. The multiple focal points 619 of the beamlets 609 impinge on multiple entrance apertures of the optical waveguide 615 or the detector element 623. The raster position and rotation angle of the entrance aperture of the optical waveguide 615 or the detector element 623 are adjusted, for example, by a translation or rotation stage 617, to the position and rotation of the raster of the focal spot 619, including spots 619.0, 619.1 to 619.4. The imaging magnification may be adjusted by a zoom lens 611.

[0059] Figure 8b shows an example of charging effects. For example, charging effects can appear at the boundaries of the exposure area, here at focused spots 619.1b, 619.3b, 619.4b, 619.5b, and 619.6b at the upper right edge of the exposure area. Such distortion of beamlets 619.1b, 619.3b, 619.4b, 619.5b, and 619.6b would result in complete signal loss for the beamlets. Furthermore, localized charging effects can distort isolated beamlets, such as those shown in beamlet 619.0b. Using the methods described above and adaptive mirror array 671, individual distortions can be compensated for. The distortions of beamlets 619.1b, 619.3b, 619.4b, 619.5b, and 619.6b are detected by monitoring system 230, which may include, for example, high-resolution CMOS camera 232 and mirror control module 820, which includes a processor programmed to continuously perform image processing of the image from CMOS camera 232 and determine the position of the focal point relative to a pre-calibrated focal position. High-resolution camera 232 has the advantage of high resolution, but also the disadvantage of a low image frame frequency, for example, less than 100 Hz. The deviations of each beamlet from its pre-calibrated focal position are converted into two tilt angles of active mirror 673, and corresponding drive signals are provided to adaptive mirror array 671. Figure 8c shows the result of the operation of adaptive mirror array 671. Focusing points 619.0c, 619.1c, 619.3c, 619.4c, 619.5c, and 619.6c are again adjusted to be at the entrance aperture of optical waveguide 615 or fast detection element 623, respectively, so that an ideal intensity signal is detected for each charged particle beamlet. Using a suitable fast detection element 623, e.g., an avalanche photodiode, a high detection frequency can be achieved, e.g., with a scanning frequency of 80 kHz.

[0060] Although the monitoring is described using an example of high-resolution sensor 232, other monitoring systems for active feedback control of the adaptive mirror array are equally feasible, for example, the number of entrance apertures of optical waveguide 615 connected to high-speed detection elements 623 may be greater than the number of third focal points, and distortion may be directly detected by the greater number of entrance apertures of optical waveguide 615 connected to the greater number of high-speed detection elements 623. For example, three or more entrance apertures of optical waveguide 615 connected to three or more high-speed detection elements 623 may be located at each ideal third focal spot. Thereby, changes in the position of the third focal spot can be detected and the tilt of the corresponding mirror of the adaptive mirror array can be derived.

[0061] The present invention is not limited to the embodiments or examples described above. The embodiments or examples may be combined with each other completely or partially, and various modifications within the scope of any person skilled in the art are also covered by the present disclosure. [Explanation of symbols]

[0062] 1. Multi-beamlet charged particle system 3 Primary charged particle beamlet or multiple primary charged particle beamlets 5 Primary charged particle beam spot 7 Object or sample 9 Secondary electron beamlet forming multiple secondary electron beamlets 13 Secondary electron beam path 15 Secondary electron focusing spot 25 Surface of object or sample 67 First Material Composition 69 Second Material Composition 100 Objective Radiation Units 101 First image plane 102 Objective Lens 103 Field Lens 108 First beam crossover 110 Collective Multi-Beam Raster Scanner 137 Electric field 151 Beam tube 153 Beam Exit Aperture 155 pipe segments 161 Coil 163 magnetic pole piece 165 Lower pole piece segment 200 detection units 205 Magnetically Driven Lens 220 Multipole corrector 222 Second Raster Scanner 225 Secondary electrons or second image plane 230 Monitoring System 232 high resolution sensor 235 Imaging Lens 237 Beam Divider Mirror 300 Charged Particle Multi-Beamlet Generator 301 Charged Particle Source 303 Collimating Lens 304 Filter Plate 305 Primary Multibeamlet Formation Unit 306 Multi-aperture plate 309 Primary Electron Beam 321 Intermediate image plane 331 First Field Lens 333 Second Field Lens 400 Beam Splitter or Divider Unit 500 sample stage 503 Specimen voltage source 505 Specimen Mounting Platform or Chuck 600 image sensor 602 Electron-Photon Conversion Unit 603 Relay Optical System 605 Condenser Lens 607 Folding Mirror 608 optical axis 609 Rays corresponding to secondary electron beamlets 611 Zoom Lens 613 Third Image Plane 615 Optical waveguide 617 Moving Stage 619 Converging Spot of Light Rays 623 Detector element 671 Adaptive Mirror Array 673 Tilted Mirror 675 Electrodes or Actuators 677 Hinges or flexures 679 Tilt angle 691 System Solution Space 800 Control Unit 810 Imaging control module 820 Mirror Control Module 830 Primary Beam Path Control Module 840 Secondary Beam Path Control Module 850 Stage Control Module 860 Scanning Control Unit 880 Control Action Processor 890 memory

Claims

1. A multi-beam charged particle beam system (1), comprising: an objective radiation unit (100) configured for illumination of a surface (25) of an object (7) arranged in a first image plane (101) with a plurality of first focused spots (5, 5.1, 5.2, 5.3) of a plurality of primary charged particle beamlets (3); a detection unit (200) configured to collect a plurality of secondary electron beamlets (9, 9.i, 9.o) excited in said plurality of first focused spots (5, 5.1, 5.2, 5.3) from said surface (25) of said object (7) and to form a plurality of second focused spots (15, 15.1, 15.2, 15.3) of said plurality of secondary electron beamlets (9, 9.i, 9.o) in a second image plane (225) of said detection unit (200); an electron-to-photon conversion unit (602) arranged in the second image plane (225) of the detection unit (200); a relay optical system (603) for imaging the light generated in said electron-to-photon conversion unit (602) and for forming a plurality of third focused spots (619, 619.0, 619.1, 619.2, 619.3, 619.4, 619.5) in a third image plane (613) of said relay optical system (603); a plurality of entrance apertures of the optical waveguide (615) or detector element (623) arranged in said third image plane (613); Equipped with 10. The multi-beam charged particle beam system of claim 1, wherein the relay optical system further comprises an adaptive mirror array and a mirror control module configured to individually control tilt angles of a plurality of mirrors of the adaptive mirror array so that positions of the plurality of third focal points are maintained constant at the plurality of entrance apertures of the optical waveguide or the detection element during image acquisition.

2. 2. The system (1) of claim 1, further comprising a monitoring system (230) including a high-resolution sensor (232), wherein the mirror control module (820) is configured to determine positional deviations of the plurality of third focal points (619, 619.0, 619.1, 619.2, 619.3, 619.4, 619.5) from the plurality of entrance apertures of the optical waveguide (615) or the detection element (623).

3. The system (1) of claim 2, wherein the mirror control module (820) is further configured to determine a tilt angle of the adaptive mirror array (671) from the position deviation.

4. The system (1) according to any one of claims 1 to 3, wherein the relay optical system (603) has a magnification of M>=20.

5. 5. The system (1) of claim 1, wherein the adaptive mirror array (671) is positioned at a distance L1 to the third image plane (613), L1>=JX×P1 / 2, where JX is the number of beamlets in one lateral direction and P1 is the pitch of third focused spots (619, 619.0, 619.1, 619.2, 619.3, 619.4, 619.5).

6. 6. The system (1) according to claim 5, wherein L1 is greater than 20 mm.

7. The system (1) according to any one of claims 1 to 6, wherein each of the plurality of mirrors (673) is an individually and continuously tiltable mirror (673, 673.1, 673.2, 673.3) comprising a flexure (677) and an actuator (675).

8. 8. The system (1) according to any one of claims 1 to 7, wherein the entrance apertures of the optical waveguide (615) or the detection element (623) are mounted on a translation stage (617) for adjustment of lateral positions or rotation angles of the entrance apertures of the optical waveguide (615) or the detection element (623) relative to the plurality of third focused spots (619, 619.0, 619.1, 619.2, 619.3, 619.4, 619.5).

9. The system (1) according to any one of claims 1 to 8, wherein the relay optical system (603) further comprises a zoom lens (611) for adjusting the magnification of the relay optical system (603).

10. A method of operating a multi-beam charged particle beam system (1), comprising: forming a plurality of first focused spots (5, 5.1, 5.2, 5.3) of a plurality of primary charged particle beamlets (3) in a first image plane (101); - focusing the plurality of secondary electron beamlets (9, 9.i, 9.o) to form a plurality of second focused spots (15, 15.1, 15.2, 15.3) of said plurality of secondary electron beamlets (9, 9.i, 9.o) in a second image plane (225); - transforming said plurality of second focused spots (15, 15.1, 15.2, 15.3) into a plurality of light rays (609, 609.1, 609.2, 609.3) and forming said plurality of third focused spots (619, 619.0, 619.1, 619.2, 619.3, 619.4, 619.5) in a third image plane (613); - individually adjusting the position of at least one third focused spot (619, 619.0, 619.1, 619.2, 619.3, 619.4, 619.5); receiving a plurality of intensity signals using a plurality of fast detection elements (623, 623.1, 623.2, 623.3); A method comprising:

11. - monitoring a plurality of positions of said plurality of third focused spots (619, 619.0, 619.1, 619.2, 619.3, 619.4, 619.5) using a high resolution image sensor (232); determining the deviation of the position of at least one third focused spot (619, 619.0, 619.1, 619.2, 619.3, 619.4, 619.5) from a predetermined position; The method of claim 10 further comprising:

12. - determining from said deviations at least one tilt angle (679.1) of a mirror (673, 673.1, 673.2, 673.3) of the adaptive mirror array (671); - individually adjusting the position of at least one third focused spot (619, 619.0, 619.1, 619.2, 619.3, 619.4, 619.5) by adjusting at least one tilt angle (679.1) of at least one mirror (673, 673.1, 673.2, 673.3) of said adaptive mirror array (671); The method of claim 11 further comprising:

13. The method according to any one of claims 10 to 12, further comprising adjusting the position or rotation angle of the plurality of high-speed detection elements (623, 623.1, 623.2, 623.3) using a translation stage (617).

14. The method according to any one of claims 10 to 13, further comprising the step of performing an image acquisition of a segment of the surface (25) of the object (7).

15. The method of claim 14 , wherein the steps are performed in parallel.

16. The system (1) of claim 1, further comprising a control unit (800) including software code for causing the multi-beam charged particle beam system (1) to perform the method of any one of claims 10 to 15.

17. The system (1) according to claim 1 or 16, wherein the mirror control module (820) is an ASIC programmed to carry out the method according to claim 11 or 12.

Images