High-resolution low-energy electron microscope and mask inspection method for providing topographic information

Abstract

This invention provides a high-resolution, low-energy electron microscope and a mask inspection method for providing topographic information. [Solution] A corrected scanning electron microscope (CSEM) and a method for operating the CSEM are presented for selectively separating material contrast from topographic contrast. The microscope and method enable high imaging resolution by backscattered electrons generated from low-energy primary electrons. The CSEM and method are applicable to mask modification and circuit editing processes having resolution requirements in the low nm range or even lower range.

Description

Technical Field

[0001] The present invention provides a scanning electron microscope having means for separating topographic information from material contrast in low-energy high-resolution imaging. The present invention is particularly applicable for inspection or monitoring of semiconductor manufacturing processes, for example, for semiconductor wafer inspection as well as for high-precision measurement applications such as mask inspection and mask correction.

Background Art

[0002] Requirements for semiconductor inspection, mask inspection, and mask correction are becoming increasingly stringent. The actual and future critical dimensions (CDs) smaller than 5 nm, smaller than 3 nm, and even smaller than 1 nm, as well as the placement of semiconductor features, are also becoming increasingly demanding. Pattern placement is usually correlated with overlay requirements typically specified by a fraction of the CD, for example, 1 / 3 or less of the CD.

[0003] Semiconductor masks usually have topography formed by an absorption structure on top of the mask material. The absorption structure is usually formed by lithography of an opaque film, for example, a chromium film up to a thickness of several tens of nm. Other materials or structures can also be used, for example, for phase-shift masks. In the case of EUV masks, similarly, other absorbers can be used, for example, as thin tantalum films or silicon nitride. In the case of high requirements for resolution and pattern or edge placement, the topographic effect becomes increasingly limiting in mask inspection and correction applications.

[0004] Computer-designed patterns on lithography masks typically have feature sizes comparable to the feature size or CD produced on the wafer. Therefore, especially in the case of EUV masks, the feature size of patterns, such as assist features for OPC (optical proximity correction), can be smaller, e.g., smaller than 10 nm, smaller than 5 nm, or even smaller than 3 nm. While the accuracy of pattern placement by optical image formation using photolithography is theoretically unlimited, in practice it is limited, for example, by the accuracy of the edge placement of features on the lithography mask. Thus, the edge placement of the mask pattern must be very well controlled within the overlay requirements of the lithography process, e.g., smaller than 3 nm, or even smaller than 1 nm, or even smaller than approximately 0.1 nm.

[0005] During semiconductor manufacturing, the requirements for resolution and accuracy increase in the same order. Typically, wafers are frequently planarized during the fabrication steps to avoid any topographic effects, such as those generated by structured layer deposition, oxidation, doping, or etching. However, some fabrication steps require inspection with increased accuracy of the topography of nearly flat features produced on the wafer.

[0006] The topographic effects of feature and mask structures during wafer fabrication limit the accuracy of conventional imaging techniques. Currently used imaging techniques involve backscattering or secondary electron collection using scanning electron microscopy. A sample, e.g., the wafer surface or mask surface, is raster-scanned with a narrowly focused primary electron beam. Secondary and backscattered electrons are collected and their intensity is evaluated. Several methods may be used to select a specific energy regime or angular spectrum of the collected electrons. However, with increasing requirements, the resolution and accuracy of currently used imaging techniques are no longer sufficient. Even shallow mask layers of tens of nanometers begin to exhibit a shadowing effect for backscattered electrons. The form and tilt angle of feature edges affect the backscattered electron signal, which is on the order of the required imaging accuracy.

[0007] In conventional scanning electron microscopes (SEMs), image intensity is typically recorded by fixed in-lens detectors located at different positions. Examples of such conventional SEMs are described in U.S. Patent No. 7,910,887 or U.S. Patent No. 10,720,304. In-lens detectors are sometimes called ESB detectors for energy-selective collection of backscattered electrons. Each detector records electrons at different emission angles or energies. Elastically backscattered electrons emitted at higher angles typically contain more topographic information, while electrons emitted near the optical axis of the microscope primarily contain material information. In the system described in U.S. Patent No. 10,720,304, primary electrons pass through the in-lens detector at the central aperture of the detector. Within this aperture, secondary or backscattered electrons cannot be detected. Therefore, information on backscattered electrons at low scattering angles is lost. This loss of information becomes increasingly limiting for low-energy electrons. At lower kinetic energies of primary electrons, the scattering angle generally decreases substantially, collecting more off-axis electrons, while electrons emitted at low and medium angles are lost. European Patent No. 2,463,889 describes a movable diaphragm aperture between an in-lens detector and an ESB detector for limiting the phase space of backscattered electrons. Such a solution is insufficient for the specification requirements described above and does not allow access to the off-axis portion of the momentum distribution of backscattered electrons. Furthermore, movable apertures are difficult to implement because the aperture needs to float to high potentials (e.g., liner tube potential). In addition, mechanical movement does not allow for rapid changes in the operating mode for high-speed image acquisition.

[0008] For high-resolution requirements, for example, during electron beam-assisted correction operations, low-energy electron microscopes are needed with electron kinetic energies lower than several hundred eV, e.g., lower than 500 eV, lower than 300 eV, or even lower. Therefore, it has been proposed to apply corrected low-energy microscopes using imaging systems to image backscattered electrons with a beam separator relative to the detector. Such systems are disclosed, for example, in U.S. Patent No. 6,855,939, U.S. Patent No. 8,592,776, and German Patent Application Publication No. 10 2019 214 936. However, these systems are configured, for example, to separate low-energy secondary electrons from backscattered electrons, but do not consider topographic information from backscattered electrons. However, for lower kinetic energies with reduced angular diffusion of collected backscattered electrons, an increasingly larger proportion of backscattered electrons contain topographic information that degrades the measurement accuracy of conventional corrected low-energy microscopes. Furthermore, material contrast information is lost, and process control applications that require material contrast can no longer be performed with sufficient accuracy.

[0009] Therefore, it is the task of the present invention to provide a high-resolution inspection system for semiconductor wafers or masks that is capable of extracting or separating entangled topographic information from material information from backscattered electrons. A further task of the present invention is to provide a high-resolution mask or semiconductor inspection method with higher accuracy and precision, taking into account topographic or edge effects of structural layer features of semiconductors or masks. A further task of the present invention is to provide a precision monitoring system for monitoring mask modification operations that meet current and future requirements for semiconductor masks. A further task of the present invention is to provide a method for fast and reliable semiconductor or mask inspection in the presence of topographic effects, and a system that can be configured for fast and reliable semiconductor or mask inspection in the presence of topographic effects. [Overview of the Initiative]

[0010] The present invention, which solves the task, is described by the independent claims. Further examples of embodiments are described in the dependent claims.

[0011] The present invention enables the application of low-energy electron imaging for investigating the surface (25) of a sample (7) using a primary electron beam having a low incident energy LE. A low-energy electron microscope (1) according to one embodiment comprises a beamforming unit (1400) configured to generate a corrected primary electron beam (3) in use. The primary electron beam (3) is pre-corrected by a correction means of the beamforming unit (1400) and focused onto the surface (25) of the sample (7) by a primary beam focusing unit (1100). The beamforming unit (1400) and the primary beam focusing unit (1100) are configured to focus the corrected primary electron beam (3) onto the surface (25) of the sample (7) and to decelerate the primary electron beam (3) before it reaches the sample surface (25). As a result, low incident energy primary electrons having kinetic energies lower than 400 eV, preferably lower than 300 eV, more preferably lower than 200 eV, or even more preferably lower than 150 eV, can be achieved with high imaging resolutions lower than 3 nm, preferably lower than 2 nm, or even lower.

[0012] A primary beam focusing unit (1100) is further configured to collect in use a backscattered electron beam (9) containing electrons scattered at a large angle from the surface (25) of the sample (7). The large angle of the collected backscattered electrons is typically greater than 0.7 radians from the perpendicular to the surface (25) of the sample (7), and preferably up to 1.3 radians. The low-energy electron microscope (1) further comprises a detection unit (1600) having at least a first confinement detector segment (1801) for detecting at least a first segment of the angular spectrum of the backscattered electron beam (9) and generating at least a first detection signal I1. The low-energy electron microscope (1) further comprises a beam splitting unit (1500) for in-use guidance of a corrected primary charged particle beam (3) from a beamforming unit (1400) to a primary beam focusing unit (1100) and for guidance of a backscattered electron beam (9) from the primary beam focusing unit (1100) to a detection unit (1600). The backscattered electron beam (9) thereby includes an axial segment of the angular spectrum of the backscattered electron beam (9) that propagates parallel to and opposite to the corrected primary charged particle beam (3). The detection unit (1600) further comprises a tuning element and is connected to a control unit (800). The control unit (800) is configured to control the tuning element to select a first selected segment of the angular spectrum of the backscattered electron beam (9) in a first imaging mode. The adjustment element comprises at least one of a deflection unit (1603) configured to deflect the backscattered electron beam (9), a focusing lens (1605) configured to focus the backscattered electron beam (9), and an adjustable energy filter (1607) that blocks backscattered electrons below the kinetic energy threshold of an adjustable dispersion unit (1611) in order to distribute the backscattered electron beam (9) according to its kinetic energy. In one example, a control unit (800) is configured to control the deflection unit (1603) to select an off-axis segment of the angular spectrum corresponding to backscattered electrons (9) scattered at a large angle from the surface (25) of the sample (7).In one example, the control unit (800) is configured to control the focusing lens (1605) to select a larger segment of the angular spectrum corresponding to backscattered electrons (9) scattered at a larger angle from the surface (25) of the sample (7). In one example, the control unit (800) is configured to control the focusing lens (1605) to select a smaller segment of the angular spectrum corresponding to backscattered electrons (9) scattered at a narrower angle from the surface (25) of the sample (7). This allows for the selection of a difference segment of the angular spectrum of the backscattered electron beam (9). In one example, the control unit (800) is further configured to control adjustment elements to select a second selected segment of the angular spectrum of the backscattered electron beam (9) that is different from the first selected segment in a second imaging mode. The control unit (800) may be further configured to sequentially perform a first image scan of a segment of the surface (25) of the sample (7) in a first imaging mode and a second image scan of the same segment of the surface (25) in a second imaging mode.

[0013] In one example, the detection unit (1600) may include a second confinement detector segment (1802) to generate a second signal I2 in use, corresponding to a second selected segment of the angular spectrum of the backscattered electron beam (9).

[0014] The control unit (800) is further configured to determine at least one of the layer edge position, feature dimensions, edge roughness, edge slope; or microdefects with an accuracy of less than 2 nm, preferably less than 1 nm, and more preferably less than 0.5 nm. High resolution is achieved by utilizing correction means of a low incident energy primary electron and low-energy electron microscope (1). Clear determination of edge position, edge slope, and microdefects is achieved by separating topographic effects from the material contrast of segments of the mask or wafer (7) surface (25). Separation is achieved by controlling the operation of a detection unit (1600) in at least first and second imaging modes, including detection of different segments of the angular spectrum of a backscattered electron beam (9). As a correction means, the low-energy electron microscope (1) includes, for example, an electrostatic mirror corrector (1415).

[0015] In one example, the control unit (800) is configured to determine at least first and second imaging modes suitable for detecting and extracting topographic effects and for separating topographic effects from the material contrast of segments on the surface (25) of the mask or wafer (7).

[0016] In a further embodiment, the low-energy electron microscope (1) comprises a beamforming unit (1400), a primary beam focusing unit (1100), a detection unit (1600) having a first confinement detector segment (1801), and a beam splitting unit (1500). The detection unit (1600) further comprises at least a second confinement detector segment (1801) for detecting at least a second segment of the angular spectrum of a backscattered electron beam (9) and generating at least a second detection signal I2 distinct from a first signal I1. The beamforming unit (1400) is configured to generate a corrected primary charged particle beam (3) in use. The beamforming unit (1400) and the primary beam focusing unit (1100) are configured to focus the corrected primary electron beam (3) onto the surface (25) of the sample (7) and to decelerate the primary electron beam (3) to a kinetic energy lower than 400 eV, preferably lower than 300 eV, more preferably lower than 200 eV, or even more preferably lower than 150 eV before reaching the sample surface (25). The primary beam focusing unit (1100) is further configured to collect backscattered electrons in use, including electrons scattered at large angles greater than 0.7 radians, preferably up to 1.3 radians, with respect to the perpendicular to the surface (25) of the sample (7).

[0017] The beam splitting unit (1500) is configured to guide the corrected primary charged particle beam (3) in use from the beamforming unit (1400) to the primary beam focusing unit (1100) and to guide the backscattered electron beam (9) from the primary beam focusing unit (1100) to the detection unit (1600). The backscattered electron beam (9) includes an axial segment of the angular spectrum of the backscattered electron beam (9) that propagates parallel to and opposite to the corrected primary charged particle beam (3).

[0018] A first confinement detector segment (1801) is configured to detect a first segment of the angular spectrum of the backscattered electron beam (9) and generate a first detection signal I1. In one example, the detection unit (1600) includes a third or further confinement detector segment (1802) to generate a third or further detection signal I3 in use, corresponding to a third or further selected segment of the angular spectrum of the backscattered electron beam (9). Thus, at least two detector segments are configured to select a plurality of different detection signals corresponding to different segments of the angular spectrum of the backscattered electron beam (9). The control unit (800) of the low-energy electron microscope (1) is further configured to determine at least one of the layer edge position, feature dimension, edge roughness, edge slope, or microdefect from at least the first and second detection signals I1 and I2 with an accuracy of less than 2 nm, preferably less than 1 nm, and more preferably less than 0.5 nm. High resolution is achieved by utilizing correction means for low incident energy primary electron and low-energy electron microscopes (1). Clear determination of edge positions, edge inclinations, and minute defects is achieved by separating topographic effects from the material contrast of segments of the surface (25) of the mask or wafer (7). Separation is achieved by at least first and second detection signals I1 and I2 corresponding to different segments of the angular spectrum of the backscattered electron beam (9). As a correction means, the low-energy electron microscope (1) includes, for example, an electrostatic mirror corrector (1415).

[0019] According to a further embodiment of the present invention, an apparatus (1000) for inspecting, correcting, or editing a mask or wafer is provided. The apparatus comprises a low-energy electron microscope (1) as described in the above embodiments. In low-energy mode, the low-energy electron microscope (1) is configured to focus a corrected primary electron beam (3) onto the surface (25) of a mask or wafer (7) in use using the low kinetic energy of primary electrons below 400 eV, preferably below 300 eV, more preferably below 200 eV, or even more preferably below 150 eV. The low-energy electron microscope (1) is further configured to collect backscattered electrons at a large angle greater than 0.7 radians, preferably up to 1.3 radians, from the perpendicular to the surface (25) of the sample (7). A beam splitting unit (1500) of the low-energy electron microscope (1) is configured to split the corrected primary charged particle beam (3) from the backscattered electron beam (9) in use. The low-energy electron microscope (1) is configured to collect and image a backscattered electron beam (9) in use, which includes an axial segment of the angular spectrum of a backscattered electron beam (9) that propagates parallel to and opposite to the corrected primary charged particle beam (3).

[0020] The detection unit (1600) of the low-energy electron microscope (1) comprises at least a tuning element. A control unit (800) of the apparatus (1000) connected to the detection unit (1600) is configured to perform an inspection task of a sample (7), for example, a mask or a segment of the surface (25) of a wafer. The detection unit (1600), having at least a first confinement detector segment (1801), is configured to selectively detect at least a first selected segment of the angular spectrum of a backscattered electron beam (9) using at least the first confinement detector segment (1801) to generate at least a first detection signal I1. The detection unit (1600) is further configured to selectively detect a second selected segment of the angular spectrum of the backscattered electron beam (9) to generate at least a second detection signal I2, wherein the second selected segment of the angular spectrum is different from the first selected segment of the angular spectrum of the backscattered electron beam (9). The control unit (800) is configured to control adjustment elements to selectively detect at least a first and / or second signal I1 and / or I2. The adjustment elements include at least one of a deflection unit (1603), a focusing lens (1605), an adjustable energy filter (1607), or an adjustable dispersion unit (1611). In one example, the control unit (800) is configured to select a single off-axis segment of an angular spectrum and perform an inspection task using the single off-axis segment of the angular spectrum.

[0021] In one example, the control unit (800) selects at least a first and / or second selected segment of the angular spectrum of the backscattered electron beam (9) based on predetermined information relating to a structure on the surface (25) of the sample (7). The structure may be an absorbing layer on a mask or a conductor wire in a wafer and may have edges or dimensions.

[0022] In one example, the control unit (800) is configured to sequentially adjust the detection unit (1600) in a first imaging mode to acquire at least a first signal I1, and then adjust the detection unit (1600) in a second imaging mode to acquire at least further signals in a subsequent second image scan across the surface (25) of the sample (7).

[0023] In one example, the detection unit (1600) includes at least a second confinement detector segment (1802) to generate, in use, at least a second or further detection signal I2 corresponding to a second or further selected segment of the angular spectrum of the backscattered electron beam (9) in a single-image scan across the surface (25) of the sample (7). The detection unit (1600) may further include a tuning element, and a control unit (800) is configured to control the tuning element to selectively detect at least first and second selected segments of the angular spectrum of the backscattered electron beam (9). An apparatus (1000) having a detection unit (1600) having at least two detector segments may be further configured to detect first and second detection signals in a first single-image scan across the surface (25) of the sample (7), and to detect third and further image signals corresponding to third and further selected segments of the angular spectrum of the backscattered electron beam (9) in a second single-image scan across the surface (25) of the sample (7) at the same inspection site.

[0024] In one example, a first selected segment of the angular spectrum of a backscattered electron beam (9) is selected to generate a first detection signal I1 having reduced sensitivity to the topography of the segment on the surface (25), and a second selected segment of the angular spectrum of the backscattered electron beam (9) is selected to generate a second detection signal I2 having increased sensitivity to the topography of the segment on the surface (25). Thereafter, the topographic information can be separated from the material contrast, and for example, the position or inclination of the layer edges can be determined with high accuracy. The apparatus according to the embodiment further comprises a plurality of gas nozzles (152) for supplying a plurality of process gases to the surface (25) of the sample (7). A control unit (800) is configured to perform at least one of an electron beam-assisted deposition or electron beam-assisted etching operation in use. The control unit (800) is further configured to start or terminate an electron beam-assisted modification or editing process based on at least the first detection signal I1 and / or the second detection signal I2.

[0025] A further embodiment of the present invention provides a method for inspecting, correcting, or editing the circuitry of a mask or wafer having a high resolution of less than 2 nm, preferably less than 1 nm, and more preferably less than 0.5 nm. The method comprises aligning an inspection area of ​​the mask or wafer (7) in the image plane (101) of a low-energy electron microscope (1). The method further comprises selecting at least a first imaging mode and a second imaging mode at the inspection area that are suitable for detecting and extracting topographic effects and for separating topographic effects from the material contrast of surface segments of the mask or wafer (7). The method further comprises a first image scan using a low incident energy primary electron beam (3) in the first imaging mode for acquiring a first image signal, and a second image scan using a low incident energy primary electron beam (3) in the second imaging mode for acquiring a second image signal. According to the method, first and second image signals are analyzed to derive topographic information and material composition of the surface segment of the mask or wafer (7) at the inspection site. In one example, during image scanning, a first signal is generated to drive adjustment elements of a detection unit (1600) for deflection and / or focusing of a backscattered electron beam (9). Thereafter, a first selected segment of the angular spectrum of the backscattered electron beam (9) is detected in a first imaging mode. In one example, a second signal is generated to drive adjustment elements of a detection unit (1600) for deflection and / or focusing of a backscattered electron beam (9). Thereafter, a second selected segment of the angular spectrum of the backscattered electron beam (9) is detected in a second imaging mode. The method further includes determining the minimum intensity (927), maximum intensity (929), width or extension dx of the shadow region, minimum intensity position Mx, and / or slope of the image signal at the layer edge (935); or at least one of the differences between a first detection signal and a second detection signal of at least one of the above values. Thereafter, at least one of the layer edge position; feature dimensions; edge roughness; edge slope; or minute defects can be determined with an accuracy of less than 2 nm, preferably less than 1 nm, and even more preferably less than 0.5 nm.The decision may be achieved, for example, by comparison with a predetermined detection signal or by model-based simulation of the detection result. For example, the method further includes the application of a machine learning algorithm with a set of training or reference data corresponding to at least one or more detection signals of layer edge location, feature dimensions, edge roughness, edge slope, or microdefects. During application to different inspection sites, the method may further receive predetermined information about the inspection site of the mask or wafer (7), and the selection of at least a first imaging mode and a second imaging mode may be performed based on the predetermined information. The predetermined information may include layer edge orientation, feature orientation, material composition, height information, or other topographic information or information about material composition.

[0026] In one example, the method includes determining and storing at least first and second imaging modes suitable for detecting and extracting topographic effects at the inspection site and for separating topographic effects from the material contrast of surface segments of a mask or wafer (7). During the determination of the ideal imaging mode, a sequence of at least two image scans is performed using a low incident energy primary electron beam (3), each of which has different selected segments of the angular spectrum of a backscattered electron beam (9) at the inspection site. At least first and second imaging modes are derived from the detection signals of the sequence. For example, the orientation and height of the layer edges are derived, and the deflection angle of the deflection unit is determined by the orientation and height of the layer edges. The first and second imaging modes are stored in memory for subsequent inspection tasks at similar inspection sites on the mask or wafer.

[0027] According to a further example, in an inspection site, at least first and second imaging modes suitable for detecting and extracting topographic effects and for separating topographic effects from material contrasts of surface segments of a mask or wafer (7) are achieved by a machine learning algorithm using a plurality of training or reference image signals. The reference or training image signals can be obtained from a calibrated reference mask or from a model-based simulation. Thereby, it is possible to identify imaging modes having a high significance for determining edge positions, feature dimensions, micro defects, edge slopes, or edge roughness with high accuracy.

[0028] After precisely determining the topography and material composition of layer edges, feature sizes, or micro defects with a resolution smaller than 2 nm, preferably smaller than 1 nm, and even more preferably smaller than 0.5 nm, an electron beam assisted correction or editing process can be triggered, initiated, or terminated.

[0029] Other advantages of embodiments of the present disclosure will become apparent from the following description in conjunction with the accompanying drawings. The present invention is not limited to embodiments and examples, but includes variations, combinations, or modifications thereof. Other embodiments of the present invention will be apparent to those skilled in the art from a consideration of the specification and practice of the invention disclosed herein. For example, the use of the apparatus and method defined in the present application is not limited to the use of an electron beam as a charged particle. Rather, any particle beam can be used, and the particle beam can induce a local chemical reaction of a precursor gas at a position where the particle beam impinges on the surface of the sample and where the corresponding gas is provided. Examples of alternative particle beams are ion beams, metal beams, molecular beams, and / or photon beams.

Brief Description of the Drawings

[0030] [Figure 1] It is a diagram showing an apparatus for mask correction according to a first embodiment of the present invention. [Figure 2] It is a diagram showing the collection of backscattered electrons in the apparatus of FIG. 1. [Figure 3] This figure shows the effect of the immersion field F on the angular distribution of backscattered electrons for primary electrons with different incident energies. [Figure 4] This figure shows the effect of different immersion fields F1 and F2 on the angular distribution of backscattered electrons. [Figure 5] This figure shows the topographic effect on backscattered electrons in a large immersion field F. [Figure 6] This figure shows the shadowing effect at the edge of an absorption layer. [Figure 7] This figure shows an example of a corrected electron microscope (CSEM) according to the present invention. [Figure 8] This figure shows an example of a primary beam focusing unit equipped with a shielding grid. [Figure 9] This figure shows a method according to a second embodiment of the present invention. [Figure 10] This figure shows an example of an imaging mode according to the present invention. [Figure 11] This figure shows three further examples of different imaging modes according to the present invention. [Figure 12] Figure 11 shows the selected segments of the angular spectra of the backscattered electron distribution for three examples of different imaging modes. [Figure 13] Figure 11 shows the detected backscattered electron intensity distributions obtained by image scanning across the layer edges, with three further examples of different imaging modes. [Figure 14] This figure shows a detection unit having multiple detector segments for a backscatter electron detector. [Figure 15] This figure shows a third embodiment of the present invention, including the operation of a distributed unit. [Figure 16] This figure shows a high-precision mask correction operation according to a fourth embodiment of the present invention, utilizing the apparatus according to the first embodiment and the method according to the second or third embodiment. [Modes for carrying out the invention]

[0031] Here, the examples are referenced in detail to the exemplary embodiments shown in the accompanying drawings. Throughout this description, the same figures in different drawings refer to the same or similar elements unless otherwise indicated.

[0032] Figure 1 shows an example of a first embodiment of the present invention. Figure 1 shows a cross-section of a mask correction apparatus 1000, which can be used to correct local defects in the absorption structure of a mask, while simultaneously preventing damage to the mask substrate during the correction process. Further details of the mask correction apparatus and method are described in U.S. Patent Application Publication 2014 / 255,831, which is incorporated herein by reference. The exemplary apparatus 1000 in Figure 1 comprises a corrected scanning electron microscope 1. The corrected scanning electron microscope 1 comprises an electron source unit 1018 for generating a primary electron beam 3. Beam imaging, beam correction, and beam deflection elements 1020 and 1025 direct the primary electron beam 3 and form a focal point of the primary electron beam 3 in the image plane (not shown) of the corrected electron microscope or CSEM 1. Further details of the CSEM 1 according to the first embodiment are described below in Figure 7.

[0033] The mask 7 is placed on the sample stage 500. The sample stage 500 is equipped with actuators and connected to a control unit, by which the mask 7 can be positioned such that the first surface 25 of the mask 7 is aligned with the image plane of the corrected electron microscope 1. The sample stage 500 may further include one or more control elements to control the temperature of the mask 7.

[0034] The exemplary apparatus 1000 uses a primary electron beam 3 as the particle beam. The electron beam 3 can be focused onto a small spot on the surface 25 of the mask 7, having a diameter smaller than 10 nanometers, for example, smaller than 5 nm or even smaller than 3 nm. The energy of the electrons striking the surface 25 of the mask 7 can vary over a certain energy range (from a few eV to 10 keV). When striking the surface 25 of the mask 7, the electrons do not cause significant damage to the mask surface 25 due to their small electron mass.

[0035] The electron beam 3 is used to scan across the surface 25 of the mask 7 to record an image of the surface 25. The detection unit 1600 provides a signal about backscatter and / or secondary electrons generated by the interaction between the primary electron beam 3 and the material of the surface 25 of the mask 7. The signal is proportional to the composition and topology of the material. Typically, the mask is made from silicon oxide, in which the absorption layer is formed of chromium or silicon nitride. EUV masks are typically made of a multilayer (MoSi multilayer) formed of molybdenum and silicon, in which the upper layer is formed of ruthenium. The absorber can be formed of, for example, silicon nitride or tantalum boron nitride. By scanning the primary electron beam across the surface 25 of the mask 7, an image of the mask surface 25 can be obtained, and defects in the absorber structure of the mask 7 can be determined. Alternatively, defects in the absorber structural elements of the mask 7 can be determined by exposing the wafer and / or by recording one or more optical images of the mask 7 determined, for example by AIMS®.

[0036] The control unit 800 includes an image forming unit (not shown) configured to control the apparatus 1000 and acquire and store an image of the surface 25 of the mask 7 acquired by the scanning operation of the primary electron beam 3. The image forming unit may be configured to execute hardware and / or software-implemented algorithms that enable the determination and modification of the image of the surface 25 of the mask 7 from the data signals of the detection unit 1600, and can store the calculated or modified image. The control unit 800 also includes a control unit for the primary beamforming unit configured to control primary beamforming, as well as beamforming and beam imaging elements 1020 and 1025. Furthermore, the control unit 800 includes a stage control unit (not shown) configured to control the movement of the sample stage 500.

[0037] The mask modification apparatus 1000 further comprises several components for manipulating or processing the mask surface 25, such as a laser system 1080 providing a laser beam 1082, an ion beam gun 1035, and several gas nozzles 152.1 to 152.6 for providing process gases. A primary electron beam 3 is used to initialize an etching or deposition reaction. The electron acceleration voltage is in the range of 0.01 keV to 10 keV. The primary electron beam current fluctuates between 1 pA and 1 nA at certain intervals. The laser system 1080 provides further and / or alternative energy transfer mechanisms with respect to the laser beam 1082. For example, the energy transfer mechanism can selectively activate a precursor gas to efficiently support a local modification process of absorber structural elements of the mask 7, or selectively activate components or fragments generated by the decomposition of the precursor gas. An exemplary apparatus 1000 comprises several storage containers 150.1 to 150.6 for different processes or precursor gases to treat one or more defects in an absorber structure located on the surface 25 of the mask 7. The first storage container 150.1 stores a first precursor gas or deposition gas to be used in combination with an electron beam 3 to generate a protective layer around defects in the absorber element, for example. The second storage container 150.2 contains a chlorine-containing etching gas by which the protective layer can be removed from the surface 25 of the mask 7. The third storage container 150.3 stores an etching gas, such as xenon difluoride (XeF2), which is used to locally remove excess absorbent material such as chromium or tantalum. The fourth storage container 150.4 stores a precursor gas for locally depositing the missing absorbent material on the surface 25 of the mask 7. The fifth storage container 150.5 and the sixth storage container 150.6 contain two further different gases which may be mixed with the etching gas stored in the third storage container 150.3 as needed. Furthermore, the apparatus 1000 may be made capable of installing additional storage containers and gas supply units as needed for selective etching or deposition for correcting mask defects.Each storage container 150.1-150.6 is connected to one of the gas nozzles 152.1-152.6 via control valves 154.1-154.6. Control valves 155.1-154.6 are connected to a control unit 800 for automated gas supply during mask correction operation. This controls the amount of gas flow supplied per unit of time, or the gas flow rate at the point where the electron beam 3 strikes the substrate 1010 of the mask 7. The distance between the point of impact of the electron beam 3 on the mask 7 and the nozzles 152.1-152.3 of the gas supply unit is within a range of a few millimeters. However, the apparatus 1000 in Figure 1 also allows for gas supply unit configurations where the distance of the gas supply unit to the point of impact of the primary electron beam 3 is less than 1 millimeter.

[0038] The mask correction apparatus 1000 further includes a pumping system configured to generate and maintain the required vacuum inside the vacuum chamber 999. Prior to starting the processing procedure, the pressure inside the vacuum chamber 999 is typically 10 -5 Pa~2·10 -4 It is within the range of Pa. At the reaction site, the local pressure can typically be increased to a maximum of approximately 10 Pa. The suction device 1085, in combination with the vacuum pump 1087, allows fragments generated by the decomposition of the precursor gas, or the portion of the precursor gas not required for the local chemical reaction, to be essentially extracted from the vacuum chamber 1090 of the apparatus 1000 at the point of generation. The suction device 1085 or the vacuum pump 1087 may be connected to the control unit 800 to control their operation. Contamination of the vacuum chamber 1090 is avoided because the unwanted gas components are locally extracted from the vacuum chamber 1090 at the incidence point of the electron beam 3 and / or laser beam 1082 before they are distributed and deposited.

[0039] The primary electron beam 3 incident on the surface 25 of the mask 7 can charge the substrate surface 25. The charge accumulation can be positive or negative. To reduce the effect of charge accumulation by the electron beam 3, a charged particle beam gun, such as an ion gun 1035, can be used to irradiate the surface 25 with ions having low energy. For example, an argon ion beam with a kinetic energy of several hundred volts can be used to neutralize the surface 25. The control unit can similarly be configured to control the ion beam source 1035. A positive charge distribution can accumulate on the surface 25 when a focused ion beam is used instead of the electron beam 3. In this case, the electron beam can be used to irradiate the surface 25 to reduce the positive charge.

[0040] It is also possible to use two or more particle beams in parallel. Laser system 1080 is incorporated into apparatus 1000 to generate laser beam 1082. Thus, apparatus 1000 can simultaneously apply electron beam 3 to mask 7 in combination with photon beam 1082. Both beams 3 and 1082 can be supplied sequentially or in pulsed form. Furthermore, the pulses of the two beams 3 and 1082 can partially overlap simultaneously or undergo intermediate reactions at the reaction site. The reaction site is the location where electron beam 3 induces a local chemical reaction of the precursor gas, either alone or in combination with laser beam 1082.

[0041] The mask correction apparatus 1000 is not limited to mask correction but can also be applied to other applications such as wafer inspection and circuit editing of fabricated wafers. In semiconductor wafer inspection, circuit editing, mask inspection, and mask correction applications, backscattered electrons are used for process control. Backscattered electrons are generated in the intersection area of ​​the primary electron beam and the sample surface, where the backscattering coefficient depends on the material at or near the surface. Typically, electron backscattering is described by several interactions with the sample surface or material, including elastic scattering, inelastic scattering, and multiple scattering processes. A simplified backscattering angle distribution is shown in Figure 2. The primary electron beam 3 of the corrected electron microscope 1 propagates in the positive z direction along the optical axis 105 of the corrected scanning electron microscope 1 (not shown in Figure 2) and is focused to an interaction volume 5 near the surface 25 of the sample 7, e.g., wafer or mask. The backscattering angle distribution 15 is largely simplified by a diffuse scattering process, which has a dominant scattering efficiency in the backward direction 12 opposite the incident electron beam 3. All angles of backscattered electrons are shown with respect to the optical axis 105 of the electron microscope 1, which is typically aligned perpendicular to the surface 25 of the sample 7. For illustrative purposes, the effects of electron diffraction in atomic or molecular lattices are thus ignored. Typically, the primary electron beam 3 is at a voltage of, for example, 8 keV to 30 keV inside the corrected scanning electron microscope 1 and is slowed by the electrostatic potential between the sample 7 and the electrode 33. The electrode 33 may be positioned between the sample 7 and the microscope 1, or inside the magnetic objective lens (not shown in Figure 2) of the corrected charged particle microscope 1. The electrode 33 may be formed, for example, by a liner tube that reaches inside the objective lens of the corrected electron microscope 1. Further electrodes may be provided between the electrode 33 and the sample 7 to further influence the electrostatic field immersion above the sample surface 25. An example of an arrangement with further electrodes is shown in U.S. Patent No. 7,910,887, which is incorporated herein by reference.

[0042] Therefore, a first voltage U1 is supplied to electrode 33, and sample 7 is set to a second voltage U2 to generate a uniform deceleration field F between the sample surface 25 and electrode 33. In one example, the sample is set to ground level, in which case the second voltage U2 = 0V. Thus, the potential difference DU = |U1 - U2| forms a field immersion field F that acts as a deceleration field F for the primary beam, thereby allowing primary electrons to reach the sample surface 25 at a lower kinetic energy or incident energy LE between 50 eV and 2 kV, but even lower kinetic energies are possible, down to LE = 0 eV. Typically, there are two different ways to control the LE of the primary electrons. In the first example, LE is controlled by changing the potential difference between electrode 33 and sample 7. In another example, the potential difference DU between electrode 33 and sample 7 is kept constant, but the kinetic energy of the primary electron beam upstream of electrode 33 is changed, for example, by a higher extraction potential of electron source 1018.

[0043] The field immersion field also acts as a boosting field F for backscattered electrons. The field immersion field F has a focusing effect on the angular distribution of backscattered electrons. Thus, in this example, the effective collection angle 19 increases relative to the microscope's acquisition angle 17, thereby still collecting backscattered electrons exiting the substrate in directions up to arrow 14 and the intermediate angle 19. Several examples of backscattered electron orbitals are shown, for example, as backscattered electron orbital 9.1 under small angles, backscattered electron orbital 9.2 under acquisition angle 19, and backscattered electron orbital 9.3 with a larger starting angle 16 than the effective collection angle 19 of the electron microscope. The collection angle 19 therefore depends on the potential difference DU. The larger the potential difference DU, the lower the kinetic energy or incident energy LE of the primary electrons on the sample surface 25, and the larger the collection angle. Typically, at an LE of about 400 eV, the collection angle 19 is about 0.4 radians (half-angle) or NA = 0.4 (where NA is the sine of the collection angle). At LE=200eV, the collection angle 19 is approximately 0.7 radians or about 40° or NA=0.64. At LE=100eV, the collection angle 19 increases to 1.3 radians or NA=0.96. At such low energies, the collection angle is 90°, containing almost all scattered electrons. The angles shown for the figure in Figure 2 correspond to an LE of approximately 200eV.

[0044] Figures 3a and 3b illustrate the focusing effect shown in Figure 2 in a typical example of the momentum distribution 27 of elastic backscattered electrons at a moderate incident energy LE of approximately 400 eV. At the dowell point 5, multiple backscattered electrons are generated, each having the same energy as the impacting primary electron, but with different directions or scattering angles. The backscattered electrons include backscattered electrons 12 parallel to the optical axis or perpendicular to the surface 25 of sample 7, and backscattered electrons 14 and 16 at moderate and large angles relative to the optical axis. Figure 3b shows the effect of the boost or immersion field F on the backscattered electrons. The momentum of the backscattered electrons increases in the direction of the boost field F, and some of the backscattered electrons are collected by the collection angle 17 of the corrected electron microscope 1. Figure 3c shows the momentum distribution 27 of elastic backscattered electrons at a low incident energy LE of approximately 200 eV. Again, at dowell point 5, multiple backscattered electrons are generated by the same, lower-energy incoming primary electrons, having different directions or scattering angles. Figure 3d shows the effect of the boost field F on the lower-energy backscattered electrons. In this example, the lower incident energy primary electrons are achieved by a lower extraction or acceleration voltage of the electron source 1018. The momentum of the backscattered electrons increases further in the direction of the boost field F, and more backscattered electrons are collected by the collection angle 17 of the corrected electron microscope 1. In this example, even 14 backscattered electrons at a moderate angle are within the collection angle 17 and are therefore collected by the corrected electron microscope 1.

[0045] Figure 4a shows the focusing effect due to different immersion fields F1 and F2. As in Figure 2, the effective backscatter angle intensity distribution 15.1 is shown as a result of acceleration by the first immersion or boosting field F1. The first immersion field F1 corresponds to the first potential difference DU1. The first boosting field F1 acts in the z direction and thus substantially extends the simplified backscatter angle distribution in Figure 2 in the z direction, forming an elliptical effective backscatter angle intensity distribution 15.1. As in Figure 2, the acquisition angle 17 (not shown) of the corrected electron microscope 1 is shown by a dashed line and corresponds to backscattered electrons below a moderate angle 14.1. The acquisition angle 17 corresponds to the acquisition angle 19 in Figure 2, where backscattered electrons below an angle 14 are collected by it. Backscattered electrons below a larger angle 16.1 are beyond the acquisition angle 17 and therefore not collected.

[0046] For even lower primary electron incident energies in the corrected electron microscope 1 with a larger second potential difference DU2, the effective in-plane moment of backscattered electrons becomes even smaller. Since the backscattered electrons are accelerated by the even larger boost potential DU2 in this case, this results in an effective extension of the backscattered electron phase space or an effective backscatter angle distribution. Figure 4b shows an example of the effective backscatter angle distribution for even lower primary electron energies, which is required for high-resolution imaging of the present invention. For even lower primary electron energies, an even larger potential difference DU2 is applied, resulting in primary electrons with even lower kinetic energy in the interaction area 5. The backscattered electrons are accelerated in the z-axis direction by a larger second boosting field F2 generated by the larger potential difference DU2, and experience an even greater focusing effect. Thus, the width of the effective backscatter angle distribution 15.2 has a further elliptical shape with a greater ellipticity given by the ratio of the major axis along the z direction to the minor axis in the x direction. As a result, backscattered electrons below angle 14 are well within the effective collection angle 17, as indicated by the backscattered electron direction 14.2. In this example, even backscattered electrons scattered at a large angle 16 are collected, as indicated by arrow 16.2. Therefore, even electrons backscattered at an angle nearly parallel to the surface 25 are collected. Similar results are obtained for the same boost field F=F1=F2 and for a decrease in the incident energy of primary electrons using a lower extraction potential, for example, at electron source 1018.

[0047] A larger effective collection angle 19 (see Fig. 2) for low-energy electrons has, on the one hand, the advantageous effect that even in the case of high-resolution imaging using low-energy electrons, more backscattered electrons are collected and a larger signal-to-noise ratio is obtained. However, the large effective collection angle 19 increases the influence of the topography effect of normal semiconductor samples such as wafers or masks. Fig. 5 shows the effect of the layer edge of a semiconductor mask during inspection. Fig. 5a shows a state with an intermediate potential difference DU1 by medium-motion-energy LE1 of primary electrons of about 300 eV to 400 eV. The primary electron beam is scanned across the surface 25 along the scanning direction 41. At the interaction position or the dovetail point 5.1, backscattered electrons are generated. All backscattered electrons within the effective collection angle 19.1 are collected and contribute to the image signal. Near the surface segment 25.1 of the substrate or the underlying layer 51, a layer edge 57 of the absorption layer 53 is provided. The absorption layer 53 has a first thickness DZ of about 15 nm to 100 nm, for example, 70 nm, and is formed, for example, by silicon nitride. The absorption layer 53 forms an edge 57 having an inclination angle 55 between 78° and 90° with the surface segment 25.1 of the first layer 51. The same situation is shown in Fig. 5b with primary electrons having a lower motion energy LE2 < LE1 when reaching the interaction volume 5. The normal incident energy of a corrected electron microscope is less than 500 eV, for example, within the range between 100 eV and 200 eV or even less than 100 eV. In a regime of less than 150 eV, a resolution of about 1 nm or even less than 1 nm, which is required for mask correction or circuit editing applications, is possible. In this example, the effective collection angle 19.2 increases as described in Figs. 2 to 4, whereby backscattered electrons at a very large angle, for example, indicated by the arrow 16, are also collected. Some of these backscattered electrons are obscured by the layer 53 where the backscattered electrons are absorbed or scattered again. This geometric effect results in a decrease in backscattered electrons, while the signal becomes smaller due to the smaller distance between the dovetail point 5 and the edge 57. For illustration, a geometric shadowing angle 21 is shown, up to which the backscattered electrons can reach the electron microscope and the detector unit.For these low-energy backscattered electrons, the backscattered electron signal depends on the distance DZ between the dowell point 5 and the edge 57, the layer thickness DZ, and the tilt angle 55 (see Figure 5a). Currently, the layer thickness DZ of the absorption layer in the semiconductor mask is approximately 60 nm to 70 nm, but further reduction to values ​​smaller than 50 nm is possible. The backscattered electron signal can be affected by the scattering of backscattered electrons at the layer edge 57 and by any charging effects that may accumulate in the layer 53 or substrate 51 due to backscattered electrons. Figure 6 shows the results for the layer edge in the backscattered electron signal. Near the layer edge 57, electrons emitted under a large polar angle in the direction of the edge 57 are lost due to shadowing or topographic effects and do not reach the detector. In the case of an intermediate LE, only a small number of backscattered electrons are collected by the smaller effective collection aperture 19.1, resulting in a lower backscattered electron signal 61 with a larger relative noise level and a lower signal-to-noise ratio (SNR). At low LEs below 400 eV, the effective collection aperture 19.2 increases significantly, collecting a greater number of backscattered electrons and resulting in a larger backscattered electron signal 63 with a higher SNR. The larger collection angle 19.2 also increases the effect of shadowing, resulting in a greater reduction in backscattered electron yield at greater distances from the layer edge, for example, up to 50 nm or even greater. Both signals 61 and 63 indicate the difference in backscattered electron yield for the two materials of the test sample (silicon dioxide vs. silicon nitride), but the low LE signal 63 in particular is closely related to strong signal disturbance from shadowing or topographic effects from the layer edge 57 (entangled). In conventional microscopes using conventional imaging techniques, the increased topographic signal 67 can reach an extension of about 50 nm, thereby making it impossible to detect mask structures at distances smaller than 50 nm, for example, with the precision required for mask correction operations.

[0048] According to the present invention, the increased topographic signal 67 in low-LE imaging is used to extract edge information, including edge position and edge inclination, e.g., inclination angle 55, within nm accuracy. The inclination angle of the attenuated phase-shift mask for the EUV mask can be, for example, between 81° and 86°. According to a further aspect of the present invention, process control of mask modification is improved by taking topographic effects into consideration.

[0049] Figure 7 shows further details of a corrected scanning electron microscope (CSEM) 1 according to a first aspect of the present invention. The CSEM 1 is suitable for low-energy backscatter electron imaging with the high resolution and high accuracy required for the mask correction and circuit editing applications described above. The corrected scanning electron microscope 1 comprises a corrected beamforming unit 1400, a beam splitter unit 1500, a primary beam focusing unit 1100, and a detection unit 1600.

[0050] The corrected beamforming unit 1400 includes an electron beam generator 1301 for generating a primary electron beam 3. The primary electron beam 3 is guided along a beam path corresponding to a first optical axis OA1. A first electrostatic lens 1403, a second electrostatic lens 1405, and a third electrostatic lens 1409 are used to collimate, focus, and image the primary electron beam 3. The condenser optical unit, consisting of lenses 1403 and 1405, may also include additional condenser lenses for adjusting the electron beam flow collected from the electron source 1301. For adjustment and control, a first electrostatic or magnetic beam deflection unit 1407 is positioned in the primary beam path. The first deflection unit 1407 may be established as a quadpole or octpole unit and may be configured for lateral adjustment, beam direction adjustment, and / or adjustment of the astigmatic shape of the primary electron beam 3.0. The first deflection unit 1407 may comprise a first multipole unit and a second subsequent multipole unit. The first and second multipole units allow the primary electron beam 3 to be aligned with the axis of the third electrostatic lens 1409 and the inlet window of the second beam deflection unit 1411. The second deflection unit 1411 deflects the primary electron beam in the direction of the second optical axis OA2, which is at an angle between 30° and 120° with respect to the first optical axis OA1. Further details of the second beam deflection unit 1411 are described in U.S. Patent No. 6,855,939, which is incorporated by reference.

[0051] The primary electron beam 3.0 is then guided along the second optical axis OA2 via a third deflection unit 1413 to an electrostatic mirror 1415. At the electrostatic mirror 1415, the primary electron beam 3 is reflected, and chromatic aberration, spherical aberration, and field curvature are corrected, at least partially, to form a corrected primary electron beam 3.1. Along its path to and from the electrostatic mirror 1415, the primary electrons 3 may further pass through at least one electrostatic lens (not shown). The corrected primary electron beam 3.1 propagates along the second optical axis OA2 and re-enters the beam deflection device 1411. In the magnetic sector of the first beam deflection device 1411, the reflected primary beam 3.1 is separated from the incident primary beam 3.0 and guided to a beam splitter unit 1500. The beam splitter unit 1500 comprises at least one magnetic sector to deflect the corrected primary electron beam 3.1 along the third optical axis 105. The beam splitter unit 1500 may comprise additional magnetic sectors or electrostatic elements. Electrons of the corrected particle beam 3.1 from the beam splitter unit 1500 propagate along the third optical axis 105 and enter the primary beam focusing unit 1100. The primary beam focusing unit 1100 focuses the corrected primary beam 3.1 onto the surface 25 of the sample 7 intended to be inspected. Within the primary beam focusing unit 1100, the corrected electron beam 3.1 is deflected by a scanning deflector 1110 and focused by an objective lens 1102 to form a corrected electron focus with a small diameter and high imaging resolution on the surface 25 of the sample 7. The objective lens 1102 may be implemented as a combination of a magnetic lens and an electrostatic lens. Correction of chromatic and spherical aberrations of the objective lens 1102 by the electrostatic mirror 1415 allows for a lower acceleration voltage of the primary electrons, and therefore a lower incident energy of the primary electrons. To achieve the low incident energy LE required for high resolution, the primary electrons of the electron beam 3.1 are decelerated by a field of immersion F (see Figure 2). Above the sample stage 500, an electrode 33 is positioned to generate the field of immersion F. In this example, the primary beam focusing unit 1100 further comprises a fixed-potential beam guide or liner tube 35 to form a free drift space for the primary electrons 3.The lower end of the beam guide tube or the beam exit end forms an electrode 33. After exiting the beam guide tube 35, the electrons of the corrected electron beam 3.1 are slowed by the immersion field to reach the potential of the sample 7. The sample 7, such as a wafer or mask, is placed on the movable sample stage 500 by a sample holder (not shown). The sample holder provides the potential U2 to the sample 7, for example, by a stage control unit 850. The potential U2 can be the ground potential or a higher potential, with U2=0. The sample stage 500 may have six actuators to position the sample surface 25 with six degrees of freedom with respect to the image plane 101 of the corrected electron microscope 1.

[0052] The immersion field F is sensitive to the charging effect on the surface 25 of the sample 7. In one embodiment, a shield electrode is attached to the primary beam focusing unit 1100. The shield electrode shields the electrostatic field due to the charging effect on the sample surface 25, and the immersion field F is unaffected. Figure 8 shows the arrangement of a shield electrode 31 in one example. The shield electrode 31 is formed by a grid made of a conductive material and is connected to a second voltage U2. The electrode material can be, for example, copper, silver, or a grid coated with copper, silver, nickel silver, or gold. The grid electrode is about 10 μm thick and has at least an aperture with a diameter of about 30 μm.

[0053] In one example, the second voltage U2 applied to the grid electrode 31 can be equal to the ground level of 0V, but other voltages are also possible. The lower end of the beam liner tube 35 forms a counter electrode 33 to which the first voltage U1 is applied. The first voltage U1 can be between 1kV and 10kV. The corrected primary electron beam 3.1 enters the beam liner tube 35 with a slightly higher kinetic energy, for example, EHT = U1 + dU, and drifts through the liner tube 35. After deceleration in the immersion field between the electrode 33 and the grid electrode 31, the primary electrons 3.1 have a kinetic energy of dU. The energy difference dU can be between 500eV and 50eV or even less. The primary electron beam 3.1 passes through the grid electrode 31 in at least one aperture. The grid electrode 31 is spaced a short distance from the sample surface, less than 20μm, preferably less than 15μm. Electrons, slowed by the immersion field between the grid electrode 31 and the counter electrode 33, are focused onto the sample surface 25 by an objective lens 1102, which is formed as a so-called axial gap lens. The objective lens 1102 is formed as a magnetic lens having at least a first coil 1104 and a yoke 1106. The upper pole shoe 1115 and lower pole shoe 1113 of the yoke 1106 form an axial gap 1108, thereby minimizing the magnetic immersion field. However, other objective lenses, such as those with a radial gap, are also possible. The primary electron focus 5 is formed on the surface of the lens and is raster-scanned by scanning deflectors 1110.1 and 1110.2. In this example, the sample 7 is connected to a third potential or third voltage U3, where U3 can be equal to or less than a second voltage U2 (U3 ≤ U2). This achieves a low incident energy less than dU. In one example, the second and third voltages U2 and U3 are both set to be equal, and the grid electrode 31 and sample 7 are both connected to ground level. The incident energy LE of the primary electrons is then equal to the difference dU of the kinetic energy of the accelerating voltage of the primary electrons 3.1 relative to the liner tube potential U1. In the third example, the third voltage or potential U3 is greater than U2.This allows the collection angle 19 to be precisely adjusted. In this example, the third voltage or potential U3 is preferably between U1 and U2, where U1 > U3 > U2.

[0054] The scanning deflector 1110 in this example is a magnetic deflector that first deflects the primary beamlet 3.1 across the surface 25 of the sample, and then helps to adjust the angle of the primary beamlet 3.1 with respect to the optical axis 105 of the primary beam focusing unit 1100, which should be positioned perpendicular to the surface 25. The primary beam focusing unit 1100 in Figure 8 further comprises a first electrostatic multipole corrector 1123 positioned between the grid electrode and the liner tube 35. Thereafter, the immersion field can be affected and beam aberrations can be minimized during scanning operations by the scanning deflectors 1110.1 and 1110.2. The first electrostatic multipole corrector 1123 may be further used to scan the primary electron beam 3 across the surface 25 of the sample 7, for example, in a high-speed scanning mode using the electrostatic poles of the first electrostatic multipole corrector 1123 during corrective operation. The first electrostatic multipole corrector 1123 can be an octave corrector or a corrector having 12 poles, for example, configured to correct astigmatism or Trefoil aberration of the primary beamlet. The second magnetic multipole corrector 1121 can be located inside the objective lens 1102 and around the liner tube 35. Such an arrangement of the primary beam focusing unit 1100 allows for very low incident energies with very high resolution. This arrangement allows for the collection of backscattered electrons in large angular spectra, enabling mask inspection or correction applications with the specification requirements discussed above.

[0055] The description of the corrected electron microscope 1 according to the first embodiment is continued here in the diagram of Figure 7. The corrected electron beam 3.1 is focused onto the sample 7 so as to enter the interaction volume 5 or dowell point 5, interacts with the sample 7, and generates secondary and backscattered electrons 9 as described above in Figures 2 and 3. The secondary and backscattered electrons are accelerated again by the magnetic immersion field as described above. The backscattered electrons are imaged by the objective lens 1102 to form an intermediate image of the dowell point 5. At the intermediate image position (not shown), the aperture 1850 may be positioned together with an in-lens detector to detect the secondary electrons. The backscattered electrons 9 pass through the aperture 1850 in the in-lens detector and enter the beam splitter unit 1500. Within the beam splitter unit 1500, the backscattered electrons propagate in the opposite direction to the corrected primary electron beam 3.1 and are therefore deflected along a different beam path. The backscattered electron beam 9 then enters the detection unit 1600. The detection unit 1600 comprises a fourth deflection unit or dispersion unit 1611, a fifth deflection unit 1603 for displacing the backscattered electron beam 9, an image-forming lens 1605, and a grid electrode 1607 acting as an electron energy filter. The detection unit 1600 further comprises an electron detector having at least a first detector segment 1801 and preferably at least a second backscattered electron detector segment 1802. The detection unit 1600 may include a sixth deflection unit (not shown) downstream of the lens 1605, by which the angle of the displaced backscattered electron beam 9 can be controlled.

[0056] After passing through the fourth deflection or dispersion unit 1611, the backscattered electron beam moves with its center of gravity aligned with the fourth optical axis OA4. The fourth deflection or dispersion unit 1611 can operate as a Wien deflector capable of controlling the deflection angle of the backscattered electrons according to their kinetic energy. The kinetic energy of the backscattered electrons may be shifted from the kinetic energy of the primary electrons by the inelastic scattering process and by the focusing power of the immersion field F. The immersion field F dispersion can accelerate any backscattered electrons in the same direction along the optical axis 105, thus increasing the kinetic energy of backscattered electrons at high scattering angles. The fourth deflection or dispersion unit 1611 can achieve a predetermined amount of dispersion correction or compensation, enabling efficient filtering of backscattered electrons with respect to their kinetic energy. The fifth deflection unit 1603 may comprise a first multipole unit and a second subsequent multipole unit for deflecting the backscattered electron beam 9. This allows backscattered electrons of a specific energy spectrum or angular spectrum to be deflected towards the backscattered electron detector 1800. In this example, the backscattered electron detector 1800 comprises a first electron detector segment 1801.1 and a second electron detector segment 1801.2. The electron lens 1605 can be a magnetic lens or an electrostatic lens. The lens 1605 allows the backscattered electron beam 9 to be defocused or focused towards the detector element for more detailed selection of the angular and energy spectra of the backscattered electron beam 9. The energy filter 1607 can be, for example, a wire mesh of a specific repulsion potential to block low-energy backscattered electrons. Such an energy filter serves as a high-pass energy filter that blocks backscattered electrons below a threshold energy. This allows for even more specific energy filtering to be achieved. The detection unit 1600 may include further elements such as an anti-scanning deflection unit to compensate for residual scanning errors of the backscattered electron beam 9.The backscattered electron beam 9 passes through the same scanning deflector 1110 as the corrected primary beam, but due to the different beam paths of the backscattered electrons, which traverse the scanning deflector 1110 at slightly different energies or angles compared to the corrected primary electron beam 3.1, it may experience slight residual scanning errors in the scanning operation of the scanning deflector 1110 compared to the primary beamlet.

[0057] However, it should be recognized that the corrected electron microscope 1 is not limited to a 90-degree deflection angle. Rather, any suitable deflection angle, e.g., between 30 and 90 degrees or even 110 degrees, can be selected by the beam deflection units 1411, 1500, and 1611, so that the first optical axis OA1 does not need to be parallel to the third optical axis 105, and the second optical axis OA2 does not need to be parallel to the fourth optical axis OA4. This example of CSEM 1 includes, for example, a mirror corrector for correcting chromatic and / or spherical aberration. However, the corrected electron microscope 1 is not limited to an SEM having a mirror corrector. Rather, other types of correction units for particle beam devices, e.g., a series of octave correctors and / or Wien filters are also possible. The energy filter 1607 is not limited to a repulsion field, but may also include a Wien filter or other energy filters.

[0058] The corrected beamforming unit 1400 is connected to a beamforming control unit 840, which is a component of the control unit 800. Further components of the control unit 800 are a stage controller 850, by which the alignment and movement of the stage 500 are controlled, and by which the sample potential 2 can be provided and controlled. The control unit 800 is further connected to the primary electron beam focusing unit 1100 by a scanning and focusing control unit 810. Further components of the control unit 800 are a detection control unit 860, by which the operation of the dispersion unit 1611, a fifth deflection unit 1603, a focusing lens 1603, and an energy filter 1607 is controlled. The image acquisition unit 880 is connected to a backscatter electron detector 1800 for acquiring the backscatter electron signal and converting that signal into digital image data.

[0059] The detection control unit 860 is configured to control the dispersion unit 1611, the fifth deflection unit 1603, the focusing lens 1603, and the energy filter 1607 in a first operating mode and in at least a second operating mode. In the first operating mode, a predetermined first angular spectral segment of backscattered electrons having a first energy spectrum is collected, and in the second operating mode, a predetermined second angular spectral segment of backscattered electrons having a second energy spectrum is collected, wherein at least the first and second angular spectra or the first and second energy spectra or both are different. The control unit 800 is further configured to selectively switch between at least two operating modes of the detection unit 1600. Thereafter, topographic information can be extracted from the angular distribution or energy distribution of backscattered electrons. In one example, the control unit 800 is configured to analyze image data acquired by at least two operating modes and to extract topographic information from the analysis. In one example, the control unit 800 is configured to analyze image data acquired separately by the first and second detector segments 1801.1 and 1801.2 and to extract topographic information from the analysis. In one example, the detector 1800 has more detector segments, e.g., four detector segments, seven or nine detector segments, or even more, for a more detailed analysis of the angular and energy spectra of the backscattered electron beam 9. Further details of the operation of the corrected electron microscope 1 and the mask correction apparatus 1000 according to the first embodiment of the present invention are described in the second embodiment of the present invention.

[0060] The masking apparatus according to the first embodiment allows full access to the backscattered angular spectrum. The detection system 1600 makes it possible to acquire information about the momentum distribution. The momentum distribution and angular spectrum are similar for backscattered electrons of similar energy. Therefore, different portions of the angular or energy distribution can be used for dynamic image acquisition for high-resolution imaging using the corrected low-energy electron microscope 1. This allows for disentanglement of material contrast from topographic information, thereby increasing the resolution and accuracy of mask inspection or mask correction applications. According to a second embodiment of the present invention, a mask inspection method for mask correction is provided. The method according to the second embodiment is shown in Figure 9.

[0061] In step S1, the inspection area of ​​the mask or wafer is aligned using the stage 500. The positions of a series of inspection areas, for example, for the mask correction process, are determined, for example, by optical image generation of the mask or from analysis of the printed wafer. In the apparatus according to the present invention, the coordinate system of the mask or wafer is then registered, the mask is aligned in the image plane 101, and the first inspection area is aligned with the optical axis 105 of the apparatus 1000 (see Figures 1 and 6 for reference). Several known methods for registration and alignment may be applied.

[0062] In step S2, at least two imaging modes are selected for high-resolution imaging of the inspection area. At least two inspection modes are selected for the detection and extraction of topographic effects and for the separation of topographic effects from material contrast effects. For each inspection mode, a set of parameters for at least one adjustment element of the detection unit 1600 is selected, including parameters for at least one of the dispersion element 1611, deflection unit 1603, lens 1605, and energy filter 1607. Each imaging mode for high-resolution imaging operates with low kinetic energy electrons having an LE < 500 eV, e.g., less than 400 eV or even less than 200 eV, e.g., 150 eV. Each imaging mode for high-resolution imaging can operate with the same LE or with different LEs, e.g., first LE1 = 200 eV and second LE2 = 150 eV.

[0063] In step S3, the first image of the examination area is acquired in the default imaging mode. The default imaging mode can be the first imaging mode for high-resolution imaging selected in step S2 or any other imaging mode.

[0064] In step S4, the image obtained from step S3 is used to determine whether a subsequent image is required using a subsequent imaging mode. The determination may include at least one of the following components: 4a) It is determined whether the image contains the expected features of the inspection site, and whether the measured expected features have the expected orientation. If there is an error in the alignment of the inspection site, the method restarts in step S1. 4b) It is determined whether a first and at least a second image have been acquired using a list of at least two different imaging modes, or whether some images from the inspection modes are missing. If further images from the inspection modes are missing, the method continues to step S5. If all images from at least two imaging modes have been acquired as selected in step S2, the method continues to step S6.

[0065] In step S5, the next inspection mode is selected from the list of at least two imaging modes determined in step S2, and the parameters of the corrected electron microscope 1 are adjusted accordingly. The imaging mode of CSEM1 is changed, and a second high-resolution imaging mode of CSEM1 with low-energy electrons is installed. Changing the imaging mode from the first high-resolution imaging mode to the second or further high-resolution imaging mode involves changing the parameters of the detection unit 1600, including parameters for at least one of the dispersion element 1611, deflection unit 1603, lens 1605, and energy filter 1607. Each imaging mode for high-resolution imaging operates with low-kinetic-energy electrons having an LE < 500 eV, e.g., less than 400 eV or even less than 200 eV, e.g., 150 eV. Each imaging mode for high-resolution imaging can operate with the same LE or different LEs, e.g., the first LE1 = 200 eV and the second LE2 = 150 eV.

[0066] The image acquisition in step S3 is repeated, and subsequent images are acquired. Steps S3 to S5 are repeated until all images are acquired using at least two imaging modes and a set of images is acquired. Once all images of the set of images selected in step S2 have been acquired, the method proceeds to step S6.

[0067] In step S6, the set of images is analyzed and at least one of the following measurement results is extracted: a) Topographic information is extracted or separated from material contrast. b) The edge position of the layer edge is determined with an accuracy of less than 2 nm, preferably less than 1 nm, and more preferably less than 0.5 nm. c) Feature dimensions are determined with an accuracy of less than 2 nm, preferably less than 1 nm, and more preferably less than 0.5 nm. d) The edge slope is determined. e) Edge roughness is determined. f) Micro-defects, such as contaminating particles, are identified.

[0068] In step S7, the extracted measurement results are analyzed, and a correction or editing process is determined and initiated, or terminated.

[0069] In Step S8, a) Whether the inspection and correction process in steps S1-S7 must be repeated on the same inspection site, or b) Whether the method can be continued at the next examination site, or c) Whether the inspection of the mask or wafer has reached completion and the method can be restarted with respect to a new mask or wafer. This is determined.

[0070] Next, the selection of the imaging mode by step S2 is described in more detail. An example of inspection according to the present invention is shown in Figure 10. Figure 10a is similar to the situation shown in Figure 5b, where, for example, a collection angle 19.2 of about 70° corresponds to a primary electron energy EHT of about 125 eV. The inspection position or dwell point 5 is at a distance DX from the edge 57 of layer 53 on the substrate 51. The thickness of layer 53 is, for example, 70 nm. Similar to the simplified diagram of the topographic effect in Figure 5b, the edge forms a shadowing region limited by the shadow angle 21. Figure 10b shows the detection unit 1600 for the first imaging mode, which is similar to the conventional imaging operation of the corrected electron microscope 1. The dispersion element 1611 is set to a standard dispersion correction. After the dispersion unit 1611, the backscattered electron beam 9 has a divergence indicated by the boundary line 919. The divergence corresponds to the angular spectrum or momentum distribution of the backscattered electron beam 9, which is symmetrically positioned with respect to the fourth optical axis OA4. The deflection system 1603 of the detection unit 1600 is in the off state. The lens 1605 is also off-stage. In this example, the detection unit 1600 further comprises an aperture stop 1613. The first detector element 1801 of the electron detector 1800 is located downstream of the aperture stop 1613, which limits the collected angular spectrum or momentum by the coordinates (px, py) of the backscattered electron beam, as indicated by angle 931. In the lower part of the figure, the collected angular spectrum of backscattered electrons is further limited by the shadow angle 921 of the layer edge 57. In this figure, the positive momentum direction px is parallel to the z axis. Figure 10c shows the collected backscattered electron distribution 925 in angular or momentum space by coordinates px and py. The figure shows the maximum collection angle 919 of the corrected electron microscope 1, where the shadow angle line 921 and the collection aperture 931 correspond to the aperture stop 1613. The collected backscattered electron signal corresponds to the integral over circle 931.

[0071] Figure 11 shows three examples of imaging modes according to the present invention. For illustrative purposes, the figure includes an acquisition angle limit 921 by aperture 1613. Figure 11a shows a first imaging mode, in which the detection unit 1600 is controlled to achieve a first deflection 603 of the backscattered electron beam 9 in the negative px direction by deflector 1603. This maximizes the topographic effect of the layer edges. Figure 11b shows a second imaging mode, in which the detection unit 1600 is controlled to achieve the focusing effect 605 of the focusing lens 1605. In this example, the focusing power of lens 1605 is adjusted so that the maximum collection angle 919 of the backscattered electron spectrum corresponds to the aperture angle of aperture stop 1613. This maximizes the collection efficiency of backscattered electrons from the backscattered electron beam 9.

[0072] Figure 11c shows the third imaging mode, where the detection unit 1600 is controlled to achieve a second deflection 607 by the deflector 1603 in the positive x-direction. This reduces the topographic effect of the layer edge 57 to a minimum. The corresponding Figures 12a-12c show the effect of the three imaging modes on the detected angular distribution 925.1-925.3 in the example of the layer edge 57 according to Figure 10a. The shadow line 921 is invariant with respect to the collected backscatter electron distribution 919. As shown in Figure 12a, the first deflection 603 in the negative x-direction allows more electrons of the backscatter electrons scattered in the positive x-direction to be collected and detected, and as a result, most of the backscatter electrons are lost due to the shadowing effect of the layer edge 57. Figure 12b shows the case where a complete backscatter electron spectrum limited by the collection angle 919 is detected. Here, the signal reaches its maximum. Figure 12c shows the effect of the second deflection 605 in the positive px direction, minimizing the influence of the topographic effect on the shadowing effect. Figure 13 compares three intensity signals of a line scan across the layer edge 57 in the x direction for three imaging modes. The first intensity signal 925.1 corresponds to the first imaging mode, the second intensity signal 925.2 corresponds to the second imaging mode, and the third intensity signal 925.3 corresponds to the third imaging mode. The collected backscattered electron signal intensity reaches a maximum intensity 929.2 in the second imaging mode 929.2. However, in the second imaging mode, a large topographic effect is superimposed across the material contrast between the backscattered electron intensity of the substrate 51 and the backscattered electron intensity of the layer 53. The topographic effect is further enhanced in the first imaging mode, and in the shadow region, the backscatter intensity almost completely disappears, with the minimum backscatter electron intensity of 927.1 reaching nearly zero. The minimum backscatter electron intensity depends on the sidewall angle 55 (see Figure 5) and can be used as a measure to determine the sidewall angle 55. In the third imaging mode, the topographic effect due to shadowing is reduced to a minimum, and the detected backscatter electron intensity of 925.3 can be superimposed by signals corresponding to the inclination angle of the layer edge or the apparent width of the sidewall.The minimum intensity of the third imaging mode, 927.3, is remarkably large and can completely disappear with large deviations from the tilt angle of 90°. The minimum position Mx3 may be slightly shifted relative to the minimum position of the second intensity, 925.2, at x=0, for example. The minimum position Mx3 also depends on the sidewall angle. The extensions of the shadow regions dx1, dx2, and dx3 differ significantly for the three imaging modes.

[0073] The selection of the imaging mode in step 2 may be carried out according to a priori information, such as the orientation of the layer edges or layer steps, or the expected tilt angle of the layer edges. The a priori information may be extracted from CAD information or from previous measurements of the mask or wafer. In these cases, the automated selection of the imaging mode may be obtained, for example, based on CAD information. In other scenarios, a first image may be acquired and analyzed, thereby determining the layer orientation and topographic effects. A further set of imaging modes may be automatically derived according to the results of the analysis. As a third option, the set of imaging modes may be selected by a user interface configured for user input. In a fourth option, the selection is carried out in default mode, and for example, at least four imaging modes are implemented, each having two deflection effects of the deflection unit 1603 in the px and py directions. Optionally, a fifth imaging mode having a focusing effect or further effect of the lens 1605 is included in the set of default imaging modes.

[0074] Step 6 evaluates a set of J intensities I(j) for a set of J imaging modes, and determines the position of the layer edge 57 and the inclination angle 55 of the layer edge 57. In the first example, the determination is obtained analytically from different values ​​of the set of J recorded intensities, for example, the width of the shadowing region dx(j), the minimum intensity value min(j) (reference figures 927.1-927.3 in Figure 13), and the minimum intensity position Mx(j). It is also possible to evaluate the inclination angle of the intensities, for example, the inclination dl(3) corresponding to the tangent of the intensity by the third imaging mode at the position of the expected layer edge 57 935.3. Further values ​​can be the maximum intensity value max(j) (corresponding to reference digits 929.1-929.3 in Figure 13) and the intensity for each imaging mode (reference digits 925.1-925.3 in Figure 13) (see reference digits 933.1-933.3 in Figure 13, left to topographic effects (left to)), which are the substrate intensity values. The values ​​can be compared to typical values ​​of layer edges obtained from reference measurements or simulation results. In the second example, the measured values ​​and further parameters for dx(j), Mx(j), inclination dI(j), maximum value Max(j), and minimum value Min(j) can be used, for example, in a model-based simulation with a geometric model having edge position and inclination angle as model parameters. In the third example, a set of J intensities I(j) can be analyzed by a machine learning algorithm, which is trained by several sets of reference intensities obtained from multiple training images of layer edges. Training strength can be obtained by measuring a calibrated reference object or by simulating a model with known structural parameters, such as a Monte Carlo simulation. Each measurement result with validated structural parameters can be sequentially appended to the training data, and the machine learning algorithm can be modified and improved on the fly.

[0075] In the method described above, image intensities from different imaging modes are acquired sequentially by the image scanning sequence. However, it is also possible to acquire different image intensities within a single image scan in which different imaging modes are sequentially performed at each dowell point 5 during the image scan. In such an example, the actuator of the detection unit 1600, for example, the deflector 1603 or the lens 1605, is preferably a high-speed electrostatic element. Another example of the present invention is illustrated in Figure 14 in which different imaging modes can be acquired in parallel in a single image scan by utilizing an electronic detector 1800 having several detector segments 1801. As shown in Figure 14a, the detection unit 1600 according to another example of the present invention comprises a detector 1800 having N detector segments 1801.1 to 1801.N. Several examples of the arrangement of the detector segments 1801 are shown in Figures 14b and 14c, for example, the quadrant detector arrangement in Figure 14b or the arrangement of four triangular detector segments in Figure 14c. However, the number of detector segments is not limited to three or four, but can be greater than four, for example, seven hexagonal detector segments or nine quadrant detector segments. Each detector segment may comprise, for example, a scintillator and an avalanche diode, but other detectors are also possible. By each detector segment 1801.j, different segments of the angular spectrum of backscattered electrons are detected within a single layer scan, as well as the intensities by the first and third imaging modes described above. With a moderate number of K, for example K ≤ 9 detector segments, the topographic signal can be extracted from K intensity signals with a sufficient signal-to-noise ratio. The signal-to-noise ratio can be further improved by summing or averaging some of the K intensity signals, for example, depending on the orientation of the topographic signal corresponding to the orientation of the layer edge 57. The image acquisition method by different imaging modes with several detector segments, as described in the example of Figure 14, can be combined as well as the actions of the deflection unit 1603, the focusing lens 1605, or the energy filter 1607. Although the energy filter 1607 is omitted in Figures 8b, 9, and 12a, it can still be present in any example of the detection unit 1600.Further deflection or convergence, or energy filtering in different modes, can generate and extract more detailed topographic information.

[0076] By detecting at least two segments of the angular spectrum of a backscattered electron beam 9 using image acquisition with at least two imaging modes of the detection unit 1600 of a corrected electron microscope (CSEM), topographic information can be separated from material contrast, and the determination of layer edges or layer slopes can be extracted with a high resolution, for example, less than 1 nm or even less than 0.5 nm. In the first example, at least two segments of the angular spectrum can be acquired by a detector segment with a limited acquisition angle in combination with a backscattered electron beam deflector 1603 or a backscattered electron beam focusing lens 1605. In the second example, at least two segments of the angular spectrum can be acquired by at least first and second detector segments. The selection of at least two segments of the angular spectrum can be enhanced by an additional energy cutoff filter 1607. In a third embodiment of the present invention, the separation of topographic information from material contrast and the determination of layer edges or layer slopes are further improved by a dispersion element 1611 of the detection unit 1600. Figure 15 illustrates the effect of the dispersion unit 1611 and the method for extracting topographic information according to a third embodiment of the present invention. Similar elements shown in Figure 7 are indicated by the same reference numerals. From a dowell point 5 on the surface 25 of the mask or wafer 7, backscattered electrons are generated and accelerated by an immersion field generated between the sample 7 and the electrode 33. The backscattered electron beam 9 is collected by the objective lens 1102, forming a first intermediate crossover 1853. An aperture 1850 may be located at or near the first crossover 1853. The backscattered electron beam 9 passes through a beam splitter 1500, forming a second crossover 1855 at the entrance of the dispersion unit 1611. Crossovers 1853 and 1855 can not be a single point for all backscattered electron trajectories, but rather are distributed over a larger volume inside the aperture stop 1850 or in close proximity to the dispersion unit 1611. The backscattered electron beam 9 is limited by the two maximum collection angles indicated by electron orbits 919.1 and 919.2.Energy separation of the backscattered electron beam 9 can be achieved by the parameters of the dispersion unit 1611. The results of the energy separation of the backscattered electron beam 9 are shown in Figure 15b. The backscattered electron beam 9 within the detection plane 1803 is distributed according to angular spectra in the px and py directions as well as an energy spectrum parallel to the px direction, and backscattered electrons with greater momentum and greater propagation angles typically have greater energy. Therefore, these backscattered electrons with larger angles corresponding to larger circles in Figure 15b are more strongly deflected by the dispersion unit 1611 in the direction of positive kinetic energy E. Figure 15b shows an example of deflection by backscattered electron energy. Other scenarios of different deflections for specific kinetic energies are also possible.

[0077] The image acquisition method described in the second embodiment can be similarly applied in combination with the third embodiment. At least two segments of the mixed angular and energy spectrum are acquired, for example, by at least two detector segments 1801.1 and 1801.2, or by any imaging mode of the imaging mode with the deflection effect of the deflector 1603 or the focusing effect of the lens 1605. The selection of at least two segments of the mixed angular and energy spectrum can be enhanced by an additional energy cutoff filter 1607. A given combination of energy dispersion and angular spectral distribution also allows for the dedicated selection of backscattered electrons that have the greatest significance for topographic effects. The selection of at least two segments of the mixed energy and angular spectrum can be acquired, for example, by optimization, for example, by standard least-squares optimization or by machine learning algorithms. For example, Monte Carlo simulation on a model object can determine at least two segments of the mixed energy and angular spectrum that have the greatest significance for the model parameters of interest. The intensities from imaging modes corresponding to at least two segments of the mixed energy and angle spectrum can be used as a training dataset for machine learning algorithms, which may later be applied in precision measurements for mask or wafer inspection, mask correction, or circuit editing applications.

[0078] Figure 16 shows a fourth embodiment of the present invention. Figure 16 shows a high-precision mask correction operation achievable using the apparatus of the first embodiment of the present invention by any method of the second or third embodiment. In the first step shown in Figure 16a, the mask defect 71.1 in the absorption line 53 on the substrate layer 51 is determined with high precision. The apparatus and method described above determine the precise extent of the defect 71.1, including at least the tilt angle 73.1 of the defect 71.1. From at least two intensities of the backscattered electron images from at least two imaging modes, and by low-energy imaging of the corrected electron microscope 1, the position, the deviation of the edge position from the target range 75, and the extent of the defect can be determined with an accuracy of less than 1 nm, preferably more than 0.5 nm. The volume of material missing from the deposited material in the correction operation can be determined with high accuracy. In the correction step, for example, the defect 71.1 is filled with chromium, for example, by utilizing low-energy electron beam-assisted deposition of the material with a precursor gas provided by apparatus 1000, to form the corrected defect 77. The performance of the correction operation is then verified by the apparatus of the first embodiment of the present invention by any method of the second or third embodiment. The resulting edge position and inclination angle 73.2 of the line 53 can be obtained with high accuracy. This ensures that the correction operation is performed very well within the specification requirements for the mask, including the stringent requirements for EUV masks having edge positions smaller than or even smaller than 0.5 nm. The correction and verification steps can also be performed iteratively. Of course, the fourth embodiment is not limited to missing material in the mask layer, but can be similarly applied to the removal of excess material in the mask layer. Furthermore, the fourth embodiment is not limited to mask correction, but can also be applied to circuit editing operations on a processed wafer. In both examples, the layer material is removed by electron beam-induced etching or deposited by electron beam-induced deposition, requiring high-precision endpoint pointing.

[0079] The present invention is generally applicable to scanning electron microscopes having a beam splitting unit that separates an incident primary electron beam from an exit backscattered electron beam. Ideally, the beam splitting unit is a corrected imaging system that preserves the angular spectral distribution of the backscattered electron beam. In a first example, the adjustment system of the detection unit, in combination with at least a first finite detector aperture, allows for dynamic adjustment of the detector acquisition angle without moving parts. The adjustment system comprises at least one of a deflection unit, an adjustable lens, an energy filter, or a dispersion unit, by which the backscattered electron distribution is deflected and filtered. The adjustment system enables the operation of a high-resolution low-energy electron microscope in different imaging modes, for example, a first mode having uniform illumination of the aperture, thereby suppressing topographic effects, or at least a second mode using different portions of the backscattered electron angular distribution to analyze the edges of layers. With further adjustable lenses, in further imaging modes, the acquisition angle may be adjusted to achieve optimal material contrast. By adjusting the detector acquisition angle, for example, the shadow width generated by layer edges can be adjusted and reduced. In a second example, the segmented or 2D detector comprises at least a second detector having at least a second finite detector aperture used to collect at least a second backscattered electron signal of different ranges of energy or momentum spectra of backscattered electrons. According to the present invention, two or more images with different imaging modes are acquired sequentially by using an adjustment unit, or in parallel by using the first and at least a second detector element. Selective access to the momentum distribution of backscattered electrons scattered nearly parallel to the sample surface allows for the acquisition of more information about the surface structure, such as the sidewalls of layer edges.

[0080] The present invention enables the parallel recording and analysis of the topography and material contrast of a sample surface by working at a larger acquisition angle and selecting at least one suitable segment of the angular spectrum and / or energy spectrum of backscattered electrons. It is further possible to extract topographic information from a single scan using a low-energy electron beam of a corrected electron microscope, utilizing low incident primary energies less than 400 eV, particularly less than 200 eV, or even less than 50 eV, thus enabling high precision and accuracy smaller than a few nanometers, e.g., less than 2 nm or even smaller, to derive, for example, height maps, layer inclination angles, or edge positions. By a suitable segment of the angular spectrum and / or energy spectrum of backscattered electrons, material contrast can be separated from topographic or shadowing effects using a suitable model. The model can be analytical or phenomenological. Sample height and material maps can then be calculated. The model can be generated, for example, by Monte Carlo simulation and compared with experimental results. The selection of appropriate segments of the angular and / or energy spectra of backscattered electrons can be based, for example, on a priori information regarding layer edges and material composition. By acquiring two or more images using different imaging modes corresponding to different acquisition angles, material contrast can be separated from topography or shadowing effects with even greater precision, or with even less a priori information, or even without any a priori information at all.

[0081] Accordingly, the present invention provides a low-energy electron microscope capable of selecting the angular spectrum and / or energy spectrum of backscattered electrons. By prior knowledge of mask materials and general structures, the inclination and position of layer edges can be measured with high precision, and endpointing in mask modification or circuit editing processes can be enabled with high precision of less than a few nanometers, e.g., less than 2 nm or even less. [Explanation of symbols]

[0082] 1. Corrected electron microscope 3. Primary electron beam 5. Interaction Area 7. Samples; wafer or mask 9 Backscattered electrons 12 Backscattered electrons parallel to the optical axis 14. Backscattered electrons at intermediate angles 15 Backscatter angle distribution 16. Backscattered electrons at a large angle 17. Capture angle 19 Effective collection angle 21 Shadowing angle 25 Sample Surfaces 27 Momentum distribution of elastic backscattered electrons 31 Grid electrodes 33 electrode 35 Liner tubes 41 Scanning direction 51 Substrate or underlayer 53 Absorption layer 55 Inclination angle 57 Edge 61 Intermediate EHT signal 63 Low EHT signal 67 Increased topographic signal 71 Defects 73. Defect angle 75 Target range of edge positions 77 Fixed defects 150 gas storage containers 152 Gas Nozzle 154 Control valve 500 stages 603 Deflection action 605 Converging effect 607 Deflection action 800 Control System 810 Scanning and Convergence Control Unit 840 Control unit for primary beamforming unit 850 Stage Controller 860 Detection and Control Unit 880 Image Acquisition Unit 919 Collection aperture for backscattered electrons 921 Backscattered electrons corresponding to shadow angle 21 925 Effective backscattered electrons detected in the first imaging mode 927 Minimum backscatter electron intensity 929 Maximum backscatter electron intensity 931 Collection angle of the first detector element 933 Strength values ​​of the substrate Slope of the intensity curve at the 935th layer edge 1000 Mask Correction Device 1020 Deflection element 1025 imaging element 1035 Ion Gun 1080 laser 1082 laser beams 1085 Suction device 1087 Vacuum pump 1090 Vacuum Chamber 1100 Primary Beam Focusing Unit 1102 Objective lens 1104 Coil 1106 York 1108 Axial Gap 1110 Scanning Deflector 1113 Lower pole piece 1115 Premium Piece 1121 First multi-pole corrector 1123 Second multi-pole corrector 1301 Particle beam generator 1400 Corrected Beamforming Unit 1403 First condenser lens 1405 Second condenser lens 1407 First deflection unit 1409 Third condenser lens 1411 Second deflection unit 1413 Third Deflection Unit 1415 Electrostatic Mirror 1500 Beam Splitter Unit 1600 detection units 1603 The fifth deflection unit 1605 Lens 1607 Energy Filter 1611 Distributed Units 1613 Aperture Top 1800 Electron Detector 1801 Detector segment 1803 Detection Plane 1850 Aperture 1853 First crossover 1855 Second Crossover

Claims

1. Apparatus (1000) for inspecting, correcting, or editing a mask or wafer, - A beamforming unit (1400) configured to generate a corrected primary charged particle beam (3) in use, - A primary beam focusing unit (1100) for focusing the corrected primary charged particle beam (3) on the surface (25) of the sample (7) with a low incident energy LE during use, and for collecting the backscattered electron beam (9) containing electrons scattered at a large angle from the surface (25) of the sample (7) during use, - A detection unit (1600) having at least a first confinement detector segment (1801) for detecting a backscattered electron beam (9), - A beam splitting unit (1500) for guiding the corrected primary charged particle beam (3) in use from the beam forming unit (1400) to the primary beam focusing unit (1100), and for guiding the backscattered electron beam (9) from the primary beam focusing unit (1100) to the detection unit (1600), - A control unit (800) connected to the detection unit (1600) and configured to perform an inspection task of the segments of the surface (25) of the sample (7), Apparatus (1000), wherein the detection unit (1600) is configured to selectively detect at least a first selected segment of the angular spectrum of the backscattered electron beam (9) using at least a first confinement detector segment (1801) to generate at least a first detection signal I1.

2. The apparatus (1000) according to claim 1, wherein the primary beam focusing unit (1100), the beam splitting unit (1500), and the detection unit (1600) are configured to collect and image the backscattered electron beam (9), which includes the axial segment of the angular spectrum of the backscattered electron beam (9), during use, the backscattered electron beam (9) propagates parallel to the corrected primary charged particle beam (3) and in the opposite direction to the corrected primary charged particle beam (3).

3. The apparatus (1000) according to claim 1 or 2, wherein the detection unit (1600) is further configured to selectively detect a second selected segment of the angular spectrum of the backscattered electron beam (9) in order to generate at least a second detection signal I2, the second selected segment of the angular spectrum being different from the first selected segment of the angular spectrum of the backscattered electron beam (9).

4. The apparatus (1000) according to any one of claims 1 to 3, wherein the detection unit (1600) comprises at least an adjustment element, and the control unit (800) is configured to control the adjustment element to selectively detect at least the first and / or second signals I1 and / or I2.

5. The apparatus (1000) according to claim 4, wherein the adjustment element comprises at least one of a deflection unit (1603) configured to deflect the backscattered electron beam (9), a focusing lens (1605) configured to focus the backscattered electron beam (9), an adjustable energy filter (1607), or an adjustable dispersion unit (1611).

6. The apparatus (1000) according to any one of claims 3 to 5, wherein the control unit (800) is configured to select a single off-axis segment of the angular spectrum and to perform the inspection task using the single off-axis segment of the angular spectrum.

7. The apparatus (1000) according to any one of claims 3 to 5, wherein the control unit (800) is configured to sequentially adjust the detection unit (1600) in a first imaging mode to collect the first signal I1, and then adjust the detection unit (1600) in a second imaging mode to collect the second signal I2 in a subsequent second image scan across the surface (25) of the sample (7).

8. The apparatus (1000) according to claim 3, wherein the detection unit (1600) comprises a second confinement detector segment (1802) for generating in use the second signal I2 corresponding to the second selected segment of the angular spectrum of the backscattered electron beam (9) in a single-image scan across the surface (25) of the sample (7).

9. The apparatus (1000) according to claim 8, wherein the detection unit (1600) further comprises at least an adjustment element, and the control unit (800) is configured to control the adjustment element to selectively detect the at least first and second selected segments of the angular spectrum of the backscattered electron beam (9).

10. Apparatus (1000) according to any one of claims 1 to 7 or 9, wherein the control unit (800) selects the at least first and / or second selected segment of the angular spectrum of the backscattered electron beam (9) based on predetermined information relating to the structure on the surface (25) of the sample (7).

11. The apparatus (1000) according to any one of claims 1 to 10, wherein the beamforming unit (1400) and the primary beam focusing unit (1100) are configured to focus the corrected primary electron beam (3) onto the surface (25) of the sample (7) using the low kinetic energy of the primary electrons, which is lower than 400 eV, preferably lower than 300 eV, more preferably lower than 200 eV, or even more preferably lower than 150 eV.

12. The apparatus (1000) according to any one of claims 1 to 11, wherein the primary beam focusing unit (1100) is configured to collect backscattered electrons at a large angle greater than 0.7 radians, preferably up to 1.3 radians, from the perpendicular to the surface (25) of the sample (7).

13. The apparatus (1000) according to any one of claims 1 to 12, wherein the at least first selected segment of the angular spectrum of the backscattered electron beam (9) is selected to generate a first detection signal I1 having reduced sensitivity to the topography of the segment of the surface (25).

14. The apparatus (1000) according to claim 13, wherein the second selected segment of the angular spectrum of the backscattered electron beam (9) is selected to generate a second detection signal I2 having increased sensitivity to the topography of the segment of the surface (25).

15. The apparatus (1000) according to any one of claims 1 to 14, further comprising a plurality of gas nozzles (152) for supplying a plurality of process gases to the surface (25) of a sample (7), wherein the control unit (800) is configured to perform at least one of electron beam-assisted deposition or electron beam-assisted etching operations in use.

16. The apparatus (1000) according to any one of claims 1 to 15, wherein the control unit (800) is further configured to start or terminate an electron beam-assisted modification or editing process based on at least a first detection signal I1 and / or a second detection signal I2.

17. A method for inspecting, correcting, or editing a mask or wafer, a) A step of aligning the inspection area of ​​the mask or wafer (7) in the image plane (101) of the low-energy electron microscope (1), b) The step of selecting at least a first imaging mode and a second imaging mode in the inspection area that are suitable for detecting and extracting topographic effects and for separating topographic effects from the material contrast of the surface segments of the mask or wafer (7), c) A step of performing a first image scan using a low incident energy primary electron beam (3) in the first imaging mode in order to acquire a first image signal, d) A step of performing a second image scan using the primary electron beam (3) with low incident energy in the second imaging mode in order to acquire a second image signal, e) The step of analyzing the first and second image signals in the inspection area in order to derive the topographic information and material composition of the surface segment of the mask or wafer (7). Methods that include...

18. Step c) is - To generate a first signal in order to drive the adjustment element of the detection unit (1600), - The method according to claim 17, further comprising deflecting and / or focusing the backscattered electron beam (9) in the first imaging mode to detect a first selected segment of the angular spectrum of the backscattered electron beam (9).

19. Step d) is - To generate a second signal to drive the adjustment element of the detection unit (1600), The method according to claim 17 or 18, further comprising deflecting and / or focusing the backscattered electron beam (9) in the second imaging mode to detect a second selected segment of the angular spectrum of the backscattered electron beam (9).

20. The method according to any one of claims 17 to 19, further comprising determining a minimum intensity (927), a maximum intensity (929), the width or extension dx of the shadow region, the minimum intensity position Mx, and / or the slope of the image signal at the layer edge (935); or at least one difference between the first image signal and the second image signal of at least one of the above values.

21. The method according to any one of claims 17 to 20, further comprising determining the edge position of a layer edge; feature dimensions; edge roughness; edge slope; or at least one of a microdefect with an accuracy of less than 2 nm, preferably less than 1 nm, and more preferably less than 0.5 nm.

22. The method according to claim 21, further comprising applying a machine learning algorithm using a set of training or reference data corresponding to at least one of the layer edge location; feature dimensions; edge roughness; edge slope; or micro-defect.

23. The method according to any one of claims 17 to 22, further comprising the step of receiving predetermined information relating to the inspection area of ​​the mask or wafer (7), wherein the selection of at least a first imaging mode and a second imaging mode is performed based on the predetermined information.

24. The method according to any one of claims 17 to 23, further comprising the step of determining and storing at least first and second imaging modes suitable for detecting and extracting topographic effects in the inspection area and suitable for separating topographic effects from the material contrast of the surface segments of the mask or wafer (7).

25. The step of determining at least the first and second imaging modes is: - A step of performing a sequence of at least two image scans using a low incident energy primary electron beam (9), wherein each of the at least two image scans has a different selected segment of the angular spectrum of the backscattered electron beam (9) at the inspection site. - A step of determining at least a first and a second imaging mode from the sequence of image scanning, - The method according to claim 24, further comprising the step of storing at least first and second imaging modes for subsequent similar examination sites.

26. The method according to claim 24 or 25, wherein determining the at least first and second imaging modes suitable for detecting and extracting topographic effects and suitable for separating topographic effects from the material contrast of the surface segments of the mask or wafer (7) is performed according to a machine learning algorithm using a plurality of training or reference image signals.

27. The method according to any one of claims 17 to 26, further comprising the step of starting or ending an electron beam-assisted modification or editing process.

28. A low-energy electron microscope (1) for investigating the surface (2) of a sample (7) using a primary electron beam corrected with a low incident energy LE, - A beamforming unit (1400) configured to generate the corrected primary charged particle beam (3) in use, - A primary beam focusing unit (1100) for in-use focusing the corrected primary charged particle beam (3) onto the surface (25) of the sample (7), and for in-use collection of a backscattered electron beam (9) containing electrons scattered at a large angle from the surface (25) of the sample (7), - A detection unit (1600) having at least a first confinement-type detector segment (1801) for detecting at least a first segment of the angular spectrum of the backscattered electron beam (9) and generating at least a first detection signal I1, - A beam splitting unit (1500) for guiding the corrected primary charged particle beam (3) in use from the beam forming unit (1400) to the primary beam focusing unit (1100), and for guiding the backscattered electron beam (9), which includes an axial segment of the angular spectrum of the backscattered electron beam (9) that propagates parallel to the corrected primary charged particle beam (3) and in the opposite direction to the corrected primary charged particle beam (3), from the primary beam focusing unit (1100) to the detection unit (1600), - The system comprises a control unit (800) connected to the detection unit (1600), The detection unit (1600) further comprises an adjustment element, A low-energy electron microscope (1) is configured to control the adjustment elements to select the at least first selected segment of the angular spectrum of the backscattered electron beam (9) in a first imaging mode, wherein the control unit (800) is configured to control the adjustment elements.

29. The low-energy electron microscope (1) according to claim 28, wherein the adjustment element comprises at least one of a deflection unit (1603) configured to deflect the backscattered electron beam (9), a focusing lens (1605) configured to focus the backscattered electron beam (9), an adjustable energy filter (1607), or an adjustable dispersion unit (1611).

30. The low-energy electron microscope (1) according to claim 28 or 29, wherein the control unit (800) is configured to control the adjustment element to select an off-axis segment of the angular spectrum corresponding to backscattered electrons (9) scattered at a large angle from the surface (25) of the sample (7).

31. The low-energy electron microscope (1) according to any one of claims 28 to 30, wherein the control unit (800) is further configured to control the adjustment element to select a second selected segment of the angular spectrum of the backscattered electron beam (9) that is different from the first selected segment, in a second imaging mode.

32. The low-energy electron microscope (1) according to claim 31, further configured to sequentially perform a first image scan of a segment of the surface (25) of the sample (7) in the first imaging mode, and a second image scan of the same segment of the surface (25) in the second imaging mode.

33. The low-energy electron microscope (1) according to any one of claims 28 to 32, wherein the detection unit (1600) comprises a second confinement detector segment (1802) for generating a second signal I2 in use corresponding to a second selected segment of the angular spectrum of the backscattered electron beam (9).

34. The low-energy electron microscope (1) according to any one of claims 28 to 33, wherein the beamforming unit (1400) and the primary beam focusing unit (1100) are configured to focus the corrected primary electron beam (3) onto the surface (25) of the sample (7), and to decelerate the primary electron beam (3) to a kinetic energy lower than 400 eV, preferably lower than 300 eV, more preferably lower than 200 eV, or even more preferably lower than 150 eV before reaching the sample surface (25).

35. The low-energy electron microscope (1) according to any one of claims 28 to 34, wherein the primary beam focusing unit (1100) is configured to collect backscattered electrons at a large angle greater than 0.7 radians, preferably up to 1.3 radians, from the perpendicular to the surface (25) of the sample (7).

36. The low-energy electron microscope (1) according to any one of claims 28 to 35, wherein the control unit (800) is further configured to determine the at least first and second imaging modes suitable for detecting and extracting topographic effects and for separating topographic effects from the material contrast of the segments of the surface (25) of the mask or wafer (7).

37. The low-energy electron microscope (1) according to any one of claims 28 to 36, wherein the control unit (800) is further configured to determine at least one of the layer edge position; feature dimensions; edge roughness; edge slope; or microdefect with an accuracy of less than 2 nm, preferably less than 1 nm, and more preferably less than 0.5 nm.

38. A low-energy electron microscope (1) according to any one of claims 28 to 37, further comprising an electrostatic mirror corrector (1415).

39. A low-energy electron microscope (1) for investigating the surface (2) of a sample (7) using a primary electron beam corrected with a low incident energy LE, - A beamforming unit (1400) configured to generate the corrected primary charged particle beam (3) in use, - A primary beam focusing unit (1100) for in-use focusing the corrected primary charged particle beam (3) onto the surface (25) of the sample (7), and for in-use collection of a backscattered electron beam (9) containing electrons scattered at a large angle from the surface (25) of the sample (7), - A detection unit (1600) having a first confinement-type detector segment (1801) for detecting a first segment of the angular spectrum of the backscattered electron beam (9) and generating a first detection signal I1, - A beam splitting unit (1500) for guiding the corrected primary charged particle beam (3) in use from the beam forming unit (1400) to the primary beam focusing unit (1100), and for guiding the backscattered electron beam (9), which includes an axial segment of the angular spectrum of the backscattered electron beam (9) that propagates parallel to the corrected primary charged particle beam (3) and in the opposite direction to the corrected primary charged particle beam (3), from the primary beam focusing unit (1100) to the detection unit (1600), The detection unit (1600) further comprises at least a second confinement detector segment (1801) for detecting at least a second segment of the angular spectrum of the backscattered electron beam (9) and generating a second detection signal I2 that is different from the first signal I1, in a low-energy electron microscope (1).

40. The low-energy electron microscope (1) according to claim 39, wherein the detection unit (1600) comprises a third confinement detector segment (1802) for generating a third signal I3 in use corresponding to a third selected segment of the angular spectrum of the backscattered electron beam (9).

41. The low-energy electron microscope (1) according to claim 39 or 40, wherein the beamforming unit (1400) and the primary beam focusing unit (1100) are configured to focus the corrected primary electron beam (3) onto the surface (25) of the sample (7), and to decelerate the primary electron beam (3) to a kinetic energy lower than 400 eV, preferably lower than 300 eV, more preferably lower than 200 eV, or even more preferably lower than 150 eV before reaching the sample surface (25).

42. The low-energy electron microscope (1) according to any one of claims 39 to 41, wherein the primary beam focusing unit (1100) is configured to collect backscattered electrons at a large angle greater than 0.7 radians, preferably up to 1.3 radians, from the perpendicular to the surface (25) of the sample (7).

43. The low-energy electron microscope (1) according to any one of claims 39 to 42, wherein the control unit (800) is further configured to determine from the at least first and second detection signals I1 and I2 at an accuracy of less than 2 nm, preferably less than 1 nm, and more preferably less than 0.5 nm, the edge position of a layer edge; feature dimensions; edge roughness; edge slope; or at least one of a microdefect.

44. A low-energy electron microscope (1) according to any one of claims 39 to 43, further comprising an electrostatic mirror corrector (1415).

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