Multi-beam imaging method with reduced impact of surface charging

Abstract

Multi-Beam Imaging method with reduced impact of surface charging A method of operating a multi-beam charged particle system with pre-charging a pre-charging area exceeding an area for image acquisition is provided. With the method, an edge effect of a charging during image acquisition is reduced.

Description

[0001] Multi-Beam Imaging method with reduced impact of surface charging

[0002] Field of the invention

[0003] The disclosure relates to a method of image acquisition with a multi-beam charged particle microscope with a reduced impact of surface charging.

[0004] Background of the invention

[0005] WO 2005 / 024881 A2 discloses an electron microscope system which operates with a multiplicity of electron beamlets for the parallel scanning of an object to be inspected with a bundle of electron beamlets. The bundle of primary charged particle beamlets is generated by directing a primary charged particle beam onto a multi-beam forming unit, comprising at least one multi-aperture plate, which has a multiplicity of openings. A portion of the electrons of the electron beam is incident onto the multi-aperture plate and is absorbed there, and another portion of the beam transmits the openings of the multi-aperture plate and thereby, in the beam path downstream of each opening, an electron beamlets is formed whose cross section is defined by the cross section of the respective opening. The plurality of primary charged particle beamlets are focused by an objective lens on a surface of a sample and trigger secondary electrons or backscattered electrons to emanate as secondary electron beamlets from the sample, which are collected and imaged onto a detector. Each of the secondary beamlets is incident onto a separate detector element or group of detector elements, so that the secondary electron intensities detected therewith provide information relating to the surface of the sample at the location where the corresponding primary beamlet is incident onto the sample. The bundle of primary beamlets is scanned systematically over the surface of the sample and an electron microscopic image of the sample is generated.

[0006] The imaging contrast of a multi-beam scanning electron microscope depends on the signal generated by secondary electrons. The yield of secondary electrons depends on material characteristics, the kinetic energy of the primary electrons and charging effects of the samplesurface. Charging effects occur at local capacities or insulators, which accumulate charges and generate deteriorating electrical fields to the primary as well as secondary electron beamlets.

[0007] Different methods have been proposed to mitigate charging effects. Flood guns are widely used to pre-charge a sample surface. An example for a multi-beam charged particle beam system with a pre-charging capability is given in US 11.239.053 BB. However, a setup comprising a flood gun is of high complexity or requires large switching times between charging and imaging.

[0008] It is therefore still a need for improvement of a mitigation of charging effects during wafer inspection. It is therefore a task of the invention to provide an improved method of operation of a multi-beam charged particle beam system, which is configured to reduce charging effects during the inspection of semiconductor samples.

[0009] Description of the invention

[0010] An improved method of operation of a multi-beam charged particle beam system is provided. The improved method of operation allows an image acquisition with increased throughput. A multi-beam charged particle beam system capable of image acquisition with reduced charging effects and increased throughput is provided.

[0011] According to an embodiment, a method of image acquisition of a surface area segment of a surface of an object is comprising pre-exposing a pre-charging surface area and acquiring an image of the surface area segment with a plurality of charge particle beamlets arranged in a raster of beamlets. According to the method, the pre-charging surface area exceeds the surface area segment by at least 1pm to 10pm, for example 5pm. In an example, during preexposing the pre-charging surface area, the object is laterally moved by a stage in a scanning pattern within a movement range between 1pm to 10pm, for example 5pm. In an example, the scanning pattern is selected according on of a circular or spiral movement of stage, or a zig-zag path of stage. In an example, during the pre-exposing, the plurality of primary charged particle beamlets is defocused with respect to a surface of the object. In an example, during the pre-exposing, an ideal focus plane of the plurality of primary chargedparticle beamlets is determined with respect to a surface of the object. In an example, during the pre-exposing, a beam property of at least one of the plurality of primary charged particle beamlets is determined. In an example, the beam property is an aberration, and the aberration is compensated by at least one electromagnetic element, for example an active multi-aperture plate, of the multi-beam system. In an example, the pre-exposing is performed such that the pre-charging surface area does exceed the surface area segment by less than 30pm, preferably by less than 20pm, even more preferably by less than 10pm. In an embodiment, a multi-beam system comprises a stage capable of lateral movement and a control unit. The control unit is comprising a control processor and a memory. The memory is comprising software instructions when executed by control processor, causing the multibeam system to perform pre-exposing a pre-charging surface area and acquiring an image of the surface area segment with a plurality of charge particle beamlets, wherein the surface area segment is comprised within the pre-charging surface area.

[0012] Embodiments of the present disclosure will be explained in more detail with reference to drawings, in which:

[0013] Figure 1 is a schematic sectional view of a multi-beam charged particle system according to an embodiment

[0014] Figure 2 illustrates an image acquisition with a plurality of primary charged particle beamlets

[0015] Figure 3 illustrates an example of an inhomogeneous charging area during an image acquisition

[0016] Figure 4 illustrates examples of a scan pattern for pre-charging with a moving stage Figure 5 illustrates an effect of the pre-charging

[0017] Figure 6 illustrates a method according to the disclosure

[0018] In the exemplary embodiments of the invention described below, components similar in function and structure are indicated as far as possible by similar or identical reference numerals. Some array elements, for example the plurality of primary charged particle beamlets, are identified by a reference number. Depending on the context, the same reference number may also identify a single element out of the array elements. Each primarycharged particle beamlet (3.1, 3.2, 3.3) is one of the plurality of primary charged particle beamlets (3).

[0019] Figure 1 is a schematic illustration of a multi-beam charged particle imaging system 1 (in short also multi-beam system 1) according to an embodiment. The multi-beam system 1 uses a plurality of charged particle beams for forming an image of an object 7. The multi-beam system 1 generates a plurality of J primary particle beams 3 which strike the object 7 to be examined in order to generate interaction products, e.g. secondary electrons, which emanate from the object 7 and are subsequently detected. The multi-beam system 1 is of the scanning electron microscope (SEM) type, which uses a plurality of primary electron beams 3 which are incident on a surface 25 of the object 7 at a plurality of locationsand generate there a plurality of primary electron beam focus spots 5. The object 7 to be examined can be of any desired type, e.g., a semiconductor wafer ora semiconductor mask, and can comprise an arrangement of miniaturized elements. The surface 25 of the object 7 is arranged in an object plane 101 of an objective lens 102 of an object irradiation unit 100.

[0020] A diameter of the minimal beam spots or focus spots 5 shaped in the object plane 101 can be small. Exemplary values of this diameter are below four nanometers (nm), for example three nm or less. The focusing of the primary charged particle beamlets 3 for shaping the focus spots 5 is carried out by the objective lens system 102. In this case, the objective lens system 102 can comprise a magnetic immersion lens. For sake of simplicity, only three primary beamlets 3.1, 3.2 and 3.3 with corresponding focus points 5.1, 5.2 and 5.3 are shown in figure 1. Exemplary values of the pitch P between the incidence locations of beam spots 5 are 1 micrometer, 10 micrometers, or more, for example 40 micrometers.

[0021] The primary particles 3 striking the object 7 generate interaction products, e.g. secondary electrons, back-scattered electrons, which emanate from the surface of the object 7. The interaction products emanating from the surface of the object 7 are shaped by the objective lens 102 to form secondary electron beamlets 9. Forsake of simplicity, through the disclosure, all the interaction products are collectively described as secondary electrons, forming secondary electron beamlets 9.

[0022] The multi-beam system 1 provides a detection beam path 13 for guiding the plurality of secondary particle beamlets 9 to a secondary electron imaging system or detection unit 200.The secondary electron imaging system 200 comprises several electron-optical lenses 205.1 to 205.5 for directing the secondary particle beams 9 towards a spatially resolving particle detector 600. The detector 600 is arranged in the image plane 225. The detector 600 is comprising a plurality of detection elements. Detection elements can for example be diodes such as PMDs, or CMOS detection elements, provided with electron-to-light conversion elements, or can be formed as direct electron detection elements.

[0023] In an example, the detector 600 comprises an electron-to-light conversion element, such as a scintillator plate, by which secondary electrons are converted into light, and a plurality of light detection elements. The combination of the electron-to-light conversion element and the plurality of light detection elements hereby form together a plurality of electron detection elements. The detector or image sensor 600 can further comprise a relay optical system for imaging and guiding the photons generated by the electron to photon conversion unit at the secondary charged particle image spots 15 on dedicated photon detection elements, such as a plurality of photomultipliers or avalanche photodiodes (not shown). Such an image sensor is disclosed in US 9,536,702, which is hereby incorporated by reference.

[0024] The imaging with the secondary electron imaging system 200 is strongly magnifying such that both the raster pitch of the primary beams on the wafer surface and the size and shape of focal points 5 of the primary beamlets 3 are imaged in much magnified fashion. By way of example, a magnification is between lOx and lOOx or more. By way of example, a magnification is between lOOx and 300x such that one nm on the wafer surface is imaged enlarged to between 100 nm and 300 nm. In an example, an image field of a multi-beam system with for example 100 pm diameter is enlarged to approximately 30 mm.

[0025] The primary particle beams 3 are generated in a charged-particle multi-beamlet generator 300 comprising at least one charged particle emitter 301, at least one collimation lens 303, a multiaperture arrangement 305 and a first field lens 331 and a second field lens 333. The charged particle emitter 301 is connected to a voltage supply for providing an emitter voltage VK to the emitter 301 and generates at least one diverging charged particle beam 309, which is at least substantially collimated by the at least one collimation lens 303, and which illuminates the multi-aperture arrangement 305. The multi-aperture arrangement 305 comprises at least one first multi-aperture or filter plate 304, which has a plurality of J openings formed thereinin a first raster arrangement. Particles of the illuminating particle beam 309 pass through the J apertures or openings of the first multi-aperture plate 304 and form the plurality J of primary beamlets 3. Particles of the illuminating beam 309 which strike the first aperture plate 304 are absorbed by the latter and do not contribute to the formation of the primary beamlets 3. A multi-aperture arrangement 305 usually has at least a further multi-aperture plate 306, for example a lens array, a stigmator array or an array of deflection elements. In this example, the particle beam 309 is perfectly collimated by collimation lens 303. However, it is also possible to design the multi-aperture arrangement 305 for a diverging or converging incident particle beam 309.

[0026] Together with the field lens 331 and a second field lens 333, the multi-aperture arrangement 305 focuses each of the primary beamlets 3 in such a way that focal points are formed in an intermediate image surface 321. Alternatively, the beam foci and the intermediate image surface 321 can be virtual. The intermediate image surface 321 can be curved and tilted to pre-compensate a field curvature and image plane tilt of the charged particle imaging system arranged downstream of the intermediate image surface 321.

[0027] The at least one field lens 103 and the objective lens 102 provide a first imaging particle optical unit for imaging the surface 321 onto the object plane 101 such that a second raster configuration of focus spots 5 of the primary beamlets is formed there. The plurality of primary charged particle beamlets 3 form a crossover point 108, in the vicinity of which a first scanning deflector 110 is arranged. The first scanning deflector 110 is used to deflect the plurality of primary beamlets 3 collectively and synchronously such that the plurality of focus spots 5 are moved simultaneously over the surface 25 of the object 7. The first scanning deflector 110 is driven by a scanning control unit 860 such that in an inspection mode of operation, a plurality of two-dimensional image data of the surface is acquired. Additionally, the multi-beam system 1 can comprise further static deflectors and multipole elements 112 configured to adjust the position and beam shapes of the plurality of the primary beamlets 3. The objective lens 102 and the projection lenses 205 provide a secondary electron imaging system 200 for imaging the object plane 101 onto the detection plane 225. The objective lens 102 is thus a lens or a lens system that is part of both the first and the second particle optical unit, while the field lenses 103, 331 and 333 belong only to the first particle optical unit 100, and the projection lenses 205 belongs only to the secondary electron imaging system 200.A beam divider 400 is arranged in the beam path of the first particle optical unit 100 between the field lens 103 and the objective lens system 102. The beam divider 400 is also part of the second optical unit in the beam path between the objective lens system 102 and the projection lenses 205.

[0028] The secondary electron imaging system 200 comprises the second, collective beam deflector 222 which is arranged in the vicinity of a crossover plane or pupil plane 21a of the secondary electron beamlets 9. The second, collective beam deflector 222 is operated synchronously with the first beam deflector 110 and compensates during use a beam deflection of the secondary electron beamlets 9 such that the focus points 15 of the secondary beamlets 9 remain at constant position on the detection plane 225. Thereby, each focus points 15 of each individual secondary beamlet 9 is kept within the area of a set of detection elements, which is assigned to the individual secondary beamlet 9.

[0029] Together with the objective lens 102, the lenses 205 serve to focus the secondary beams 9 on the spatially resolving detector 600 and, in the process, compensate the imaging scale and the twist of the plurality of secondary electron beamlets 9 as a result of a magnetic lens such that a third raster arrangement of the focal points 15 of the plurality of secondary electron beamlets 9 remains constant on the detector plane 225. The electron-optical lenses 205.1 to 205.5 are shown as magneto-optical elements but are not limited to magneto-optical elements and can comprise also electro-static lens elements or stigmators. The secondary electron imaging system 200 further comprises an exchangeable contrast aperture 284 at a second pupil plane 21b of the secondary electron imaging system 200.

[0030] Further information relating to such multi-beam particle beam systems and components used therein, such as, for instance, particle sources, multi-aperture plate and lenses, can be obtained from the international patent applications WO 2005 / 024881, WO 2007 / 028595, WO 2007 / 028596, WO 2011 / 124352 and WO 2007 / 060017 and the German patent applications having the publication numbers DE 10 2013 016 113 Al and DE 10 2013 014 976 Al, the disclosure of which in the full scope thereof is incorporated by reference in the present application.

[0031] The multi-beam charged particle imaging system 1 furthermore comprises a control unit 800 configured both for controlling the individual particle optical components of the multipleparticle beam system and for evaluating and analyzing the signals obtained by the detector 600. In this case, the control or controller unit 800 can be constructed from a plurality of individual electronic components, devices and computers. By way of example, the control unit 800 comprises a control operation processor 880, a control module 840 for the control of the electron-optical elements of the secondary electron imaging system 200 and a control module 830 for the control of the electron-optical elements of the primary beamlet generation unit 100. The control unit 800 further comprises a stage control module 850 for positioning and moving the sample surface 25 or sample 7 by stage 500 within the object plane 101. The control unit 800 further comprises a control module 503 to adjust a sample voltage VS, and for supplying the sample voltage VS to the sample 7.

[0032] Further, the control unit 800 comprises the scanning control module 860. The plurality of intensity signals of the plurality of secondary electron beamlets 9 is transferred to the image data acquisition unit 810, where the image data is processed and stored in memory 890. The setup of the secondary electron optical imaging system 200, the detector 600, and the assignment of the sets of detection elements to the focus spots 15 of the secondary electron beamlets 9 is initially determined and stored in the memory 890 of the control unit 800 of the multi-beam charged particle imaging system 1.

[0033] The plurality of focus spots 5 of the primary beamlets 3 form a regular raster arrangement of incidence locations, which are formed in the object plane 101. For a large throughput, a large number J of beamlets 3 is desired. A large number J of beamlets 3 can be achieved with a hexagonal arrangement of beamlets 3, wherein the plurality of beamlets 3 is arranged on a hexagonal grid within an enclosing hexagon. The number J of beamlets 3, and hence the number of incidence locations or focus spots 5, can be for example J = 91, J = 127 or J = 469.

[0034] Figure 2 illustrates examples of an image acquisition with the plurality of primary charged particle beamlets 3. For example, the scanning operation control module 860 is configured to provide during use a scanning signal to scanning deflector 110. Thereby, each primary charged particle beamlet 3 is deflected by the collective multi-beam raster scanner 110 such that the corresponding focus spot 5.i is scanned over an image patch 245. i of a single beamlet (Figure 2a). Each image patch 245. i has a diameter AP of for example 8pm to 10pm, which is corresponding to the pitch p of beamlets 3. The scanning operation comprises a scanning of a plurality of parallel image scanning lines 241 along scanning direction 143.1 for imageacquisition. At the end of each image scanning line 241, each beamlet 3 is moved back to the starting position of a next scanning line, which is also called "flyback" 243. During image acquisition along image scanning lines 241, the scanning operation is controlled to achieve a dwell time dT between 20ns to 50 ns at each image point, with for example 8000 images points per image scanning line 241. The time for flyback 243 can be much shorter, for example 20ns in total. Figure 2b shows the parallel operation of a plurality of primary charged particle beamlets 3 to acquire an image of a surface area segment 251 of a wafer surface, consisting of a plurality of image patches 245 arranged in a pattern like the raster of beamlets. Figure 3 illustrates a situation during an image acquisition of a surface area segment 251 of a wafer surface 25 with the raster 255 of beamlets 5. During scanning operation, a plurality of image stripes 253 are exposed by the focus points 5 of primary electron beamlets 3. Depending on the material composition at the surface 25, the image stripes 253 experience a surface charging, which deteriorates the image acquisition. Due to a charging effect, primary electron beamlets 3 might be deflected and secondary electrons might leave the sample at larger angles and thus not passing aperture 284 anymore. This effect of image deformation and contrast reduction is especially large at the outer circumference 257 of surface area segment 251.

[0035] According to an embodiment of the invention, the effect of image deformation and contrast reduction at the outer circumference 257 is reduced by a local pre-charging. An example is illustrated in figure 4. A local pre-charging is achieved by pre-exposing the wafer 7 at an inspection site before the image acquisition. The pre-charging area 263 is selected to exceed the surface area segment 251 to be acquired by the plurality of beamlets 3. The local precharging of the pre-charging area 263 is achieved by moving the wafer 7 by stage 500, for example in a spiral of circular path 261, while exposing the wafer surface 25 with the plurality of primary charged particle beamlets 3. Figure 5 illustrates an effect of the pre-charging area 263. Figure 5a illustrates the image of the raster 255a of beamlets without pre-charging. Due to the charging effect, especially the peripheral beamlets at the boundary 257 (see also figure 4) of the raster 251 of beamlets are deteriorated. Figure 5b illustrates the image of the raster 255b of beamlets with pre-charging with pre-charging area 263 exceeding the surface area segment 251. The local pre-charging is achieved by exposing the pre-charging area 263 with primary electron beamlets 3 and scanning the beamlets 3 over the pre-charging area 263, thereby creating a surface charge at the pre-charging area 263 with a size larger than thesurface area segment 251 by for example 1pm, 5pm or more, for example 10pm. Thereby, an edge effect of a surface charge during image acquisition is reduced and the peripheral beamlets at the boundary 257 of the raster 255 of beamlets 3 are not deteriorated. The scanning of primary beamlets 3 can be achieved by moving or stepping the wafer stage 500 in a circular or spiral path, or any other movement pattern within a small movement range of about 1pm, 5pm or more, for example 10pm. In an example, pre-charging by the scanning exposure is assisted by defocusing the primary beamlets 3, such that the focus spots 5 are enlarged. Defocusing can either be achieved by electromagnetic lenses within the object irradiation unit 100 or by axial movement of stage 500 in z-direction. The time required for pre-charging is reduced by reducing the pre-charging area 263 to exceed the surface area segment 251 by not more than 30pm, for example by not more than 20pm, for example by 10pm or even less, for example by 5pm.

[0036] Figure 6 illustrates a method according to the disclosure. In step INIT, an image acquisition is initialized. The inspection position on a wafer surface 25 is moved within the position of the raster 251 of primary beamlets, and any pre-exposure or previous image acquisitions is determined. A pre-charging area 263 is determined, such that it is exceeding the target area of an image segment 251 to be acquired. During the determination, any pre-exposure or previous image acquisitions may be considered. According to the pre-charging area 263, a movement range and scanning pattern of a wafer stage 500 is determined. A scanning pattern can comprise any of a circular, a spiral, or a zig-zag movement path of wafer stage 500. A movement range is for example selected between 1pm and 5pm.

[0037] In Step PREEXP, the wafer surface 25 is pre-exposed by primary charged particle beamlets 3 during scanning operation by scanning movement of the wafer stage 500 according to the scanning pattern and limited by the small movement range of about 1pm to 10pm, for example 5pm.

[0038] In Step IA, an image of the surface segment 251 is acquired. During image acquisition step IS, the plurality of primary beamlets 3 is scanned by deflection scanner 110 over the surface segment 251, and secondary electrons are detected to acquire information about local material composition of the wafer surface 25.During step PREEXP, the primary beamlets 3 can be defocused, i.e. the focus spots 5 with minimum diameter can be displaced from the surface of the object. In another example, during step PREEXP, further information can be acquired. For example, during step PREEXP, an ideal focus plane of the primary beamlets 3 can be determined and adjusted. For example, during step PREEXP, individual properties of the primary beamlets 3 can be determined and aberrations can be compensated by an active multi-aperture plate 306. For example, during step PREEXP, a large beam current of primary charged particle may be used.

[0039] With the method according to the embodiment, an effect of a surface charge, especially an effect of an inhomogeneous surface charging, is reduced without a large throughput loss as with conventional methods. An effect of an inhomogeneous surface charging is reduced by pre-charging a pre-charging area only slightly exceeding the area for image acquisition.

[0040] The invention can be applied to wafer inspection but is not limited thereto. Other objects for inspection can for example be lithography masks, material science objects, or life-science objects such as tissue.

Claims

Claims1. A method of image acquisition of a surface area segment (251) of an object (7) with a plurality of charge particle beamlets (3) arranged in a raster of beamlets (255), the method comprising- pre-exposing a pre-charging surface area (263),- acquiring an image of the surface area segment (251),wherein the pre-charging surface area (263) exceeds the surface area segment (251) by at least 1pm to 10pm, for example 5pm.

2. The method according to claim 1, wherein during pre-exposing the pre-charging surface area (263), the object (7) is laterally moved by a stage (500) in a scanning pattern within a movement range between 1pm to 10pm, for example 5pm.

3. The method according to claim 2, wherein the scanning pattern is selected according to one of a circular or spiral movement of the stage (500), or a zig-zag path of the stage (500).

4. The method according to any of the claims 1 to 3, wherein the pre-exposing is performed such that the pre-charging surface area (263) exceeds the surface area segment (251) by less than 30pm, preferably by less than 20pm, even more preferably by approximately 10pm.

5. The method according to any of the claims 1 to 4, wherein during the pre-exposing, the plurality of primary charged particle beamlets (3) is defocused with respect to a surface (25) of the object (7).

6. The method according to any of the claims 1 to 5, wherein during the pre-exposing, an ideal focus plane of the plurality of primary charged particle beamlets (3) is determined with respect to a surface (25) of the object (7).

7. The method according to any of the claims 1 to 6, wherein during the pre-exposing, a beam property of at least one of the primary charged particle beamlets (3) is determined.

8. A multi-beam system (1) with a control unit (800) comprising a control processor (880) and a memory (890), the memory (890) comprising software instructions when executed by control processor (880), causing the multi-beam system (1) to perform any of the methods of claims 1 to 7.

9. The multi-beam system (1) of claim 8, further comprising a stage (500) with a stage control unit (850) configured for executing a scanning pattern according to one of a circular or spiral movement of the stage (500), or a zig-zag path of the stage (500) within a movement range between 1pm to 10pm, for example 5pm.

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