Electron beam system for inspection and review of 3D devices

Abstract

To provide improved systems and methods for generating electron beams.SOLUTION: An electron beam system for wafer inspection and review of 3D devices provides a depth of focus up to 20 microns. To inspect and review wafer surfaces or lower-sub-micron surface defects with low landing energies in hundreds to thousands of electron volts, a Wien-filter-free beam splitting optics with three magnetic deflectors can be used with an energy-boosting upper Wehnelt electrode to thereby reduce spherical and chromatic aberration coefficients of an objective lens.SELECTED DRAWING: Figure 6(a)

Description

[Technical field]

[0001] The present disclosure relates to electron beam systems. [Background technology]

[0002] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to a provisional patent application, having U.S. Application No. 63 / 010,097, filed April 15, 2020, the disclosure of which is incorporated herein by reference.

[0003] The evolution of the semiconductor manufacturing industry places increasing demands on yield management, especially on metrology and inspection systems. As feature line widths (Critical Dimensions, CDs) continue to shrink, the time required to achieve high yields and high value manufacturing must also be reduced. Minimizing the total time it takes to detect and correct a yield problem determines the return on investment for semiconductor manufacturers.

[0004] The fabrication of semiconductor devices, such as logic devices and memory devices, typically involves subjecting a single semiconductor wafer to multiple manufacturing processes to form the various functions and levels of the semiconductor device. For example, lithography is a semiconductor manufacturing process that involves the transfer of a pattern from a reticle to a photoresist disposed on a semiconductor wafer. Other examples of semiconductor manufacturing processes include, but are not limited to, chemical mechanical polishing (CMP), etching, deposition, and ion implantation. An array of multiple semiconductor devices fabricated on a single semiconductor wafer may be separated into individual semiconductor devices.

[0005] In order to increase the yield of the manufacturing process and increase profits, inspection processes are used at various stages of semiconductor manufacturing to detect defects on wafers. Inspection has always played a key role in the manufacturing of semiconductor devices, such as integrated circuits (ICs). However, as the dimensions of semiconductor devices shrink, inspection becomes even more important to successfully manufacture qualified semiconductor devices, since smaller defects can cause device failures. For example, as the dimensions of semiconductor devices shrink, detection of even smaller defects becomes necessary, since even relatively small defects can cause undesirable deviations in the semiconductor device.

[0006] However, as design rules shrink, semiconductor manufacturing processes may be operating closer to their performance limits. In addition, as design rules shrink, smaller defects may affect the electrical parameters of a device, making inspection more sensitive. As design rules shrink, the number of potentially yield-relevant defects detected by inspection increases dramatically, and the number of harmful defects detected by inspection also increases dramatically. Thus, more defects may be detected on a wafer. Modifying the process to eliminate all defects may be difficult and costly. Once the defects that actually affect the electrical parameters and yield of a device are determined, process control methods can focus on those defects and largely ignore the rest. Furthermore, with smaller design rules, process-induced defects sometimes tend to be systematic. That is, the process-induced defects tend to occur at certain design patterns. These design patterns are often repetitive patterns that are included in large numbers within a design. Eliminating spatially systematic, electrical-related defects may affect yield.

[0007] Focused electron beam systems are commonly used to create or inspect the microstructure of components such as silicon wafers used in the manufacture of integrated circuits. The electron beam is formed by electrons emitted from an emitter in an electron gun and acts as a fine probe as it interacts with the wafer to inspect the microstructure.

[0008] The 2D planar process of nanofabrication attempts to keep up with the development of semiconductor chips, but faces bottlenecks due to the physical effects of increasing IC integration density. To address the shortcomings of the 2D process, 3D volumetric processes of chip fabrication have been developed. An important part of the development of 3D nanofabrication processes is the design and fabrication of 3D NAND flash memory. Figure 1 shows a schematic diagram of a 3D NAND flash memory. Figure 2 is a cross-sectional view of Figure 1, modeling stacked bits and memory holes in a 3D NAND flash memory device. A 3D NAND flash device includes a number of stacked thin films (e.g., Si and SiO2 thin films) formed on a substrate, and memory holes (or channel holes). The Si thin film together with the SiO2 thin film forms one layer of the NAND device, and one layer of this stacked thin film together with one memory hole forms one bit of the NAND flash. Currently, 96-layer NAND flash is on the market, and 128-layer and up to 256-layer NAND flash devices are expected to be on the market in the near future.

[0009] A 96-layer NAND requires 192 thin films of Si and SiO2. A commonly used 8GB NAND flash requires about 83 million memory holes (8e9 / 96=8.3e7). Looking at a complete wafer, there are hundreds of billions of memory holes. All of these memory holes are created by nano-etching technology, as shown in Figure 2. If the etching result is good, a memory hole like that in Figure 2(c) should be formed. However, due to the limitations of the technology, highly uniform etching results of hundreds of billions of memory holes with a depth of hundreds of layers cannot always be obtained. For example, some memory holes may be over-etched or under-etched, as in Figure 2(a) and (b). Other memory holes may remain defective after etching or be etched at an angle, as in Figure 2(d) or (e), respectively. Therefore, wafer inspection and review by electron beam of these hundreds of billions of memory holes may be required to develop 3D NAND flash memory.

[0010] Figure 3 shows a model of inspecting a 3D NAND memory hole using a focused electron beam. The memory hole in 3D NAND flash is characterized by a very high aspect ratio (AR). AR is defined as the ratio of the hole diameter D to the hole depth H. For example, if D=0.1μm and H=20μm, then AR=1:200. The value of H=20μm means that for a 100-layer NAND flash, each thin film is only 0.1μm thick.

[0011] Optically speaking, a very high AR translates into a large depth of focus (DOF) for the electron beam system used to perform wafer inspection and review. As shown in FIG. 3, to accommodate an embodiment with an AR of 1:200, the DOF of the focused electron beam may need to be greater than 20 μm to ensure that all electrons in the beam hit the bottom of the memory hole. In other words, if d(z) in FIG. 3 is the electron beam spot size from the top to the bottom of the memory hole, then d(z) should be less than D throughout the entire depth H.

[0012] E-beam systems with DOFs of tens of microns are difficult to design and implement, however, for wafer inspection, review, and linewidth (CD) metrology, large DOF e-beam systems can be used not only for 3D NAND flash, but also for 3D dynamic random access memory (DRAM) electronics and all other high aspect ratio (HAR) devices (e.g., deep trench logic devices).

[0013] Photon beams are not typically used for inspection and review of HAR devices because the numerical aperture (NA) of a focused photon beam is rather large (e.g., up to tens of degrees). The NA is equal to the beam half angle β in Figure 3. The DOF of a focused beam is inversely proportional to the NA, and therefore the DOF of a photon beam is very short.

[0014] E-beam systems use secondary electrons (SE) as signals to perform and / or complete wafer inspection, review, and CD measurements. However, secondary electron signals can only reflect the characteristics from the surface of the material. Secondary electrons cannot escape from the bottom of the HAR memory hole due to their low energy (e.g., a few electron volts) and large emission polar angle, which causes them to be stopped by the wall of the memory hole.

[0015] E-beam systems also use backscattered electrons (BSEs) as a signal to inspect and review the bottom of a contact hole in a DRAM with a relatively high AR, or the bottom of a memory hole with tens of thin film layers, as shown in FIG. 4. In FIG. 4, a primary beam of electrons with a landing energy (LE) typically in the range of about 1 keV to 20 keV strikes the bottom of the memory hole. BSEs with the same energy as the landing energy are generated and, due to their relatively high energy and large emission polar angle, penetrate the thin film material stack and escape from the top surface of the material. The BSEs that escape by penetrating the thin film material stack can be collected by one or more detectors located in the electron beam optical column on the wafer.

[0016] Conventional e-beam systems that use BSE signals to inspect and / or review HAR devices have drawbacks. First, such systems are limited in their use. The e-beam voltage (or beam energy) is typically limited to less than 35 kV or less than 25 kV, or the landing energy of the e-beam on the wafer is typically limited to less than 30 keV. Thus, the BSE energy is also typically limited to less than 30 keV. Thus, the deposited thin film material cannot be made thicker, which would result in insufficient power of the BSE to penetrate and exit the bulk of the material. This limits such e-beam systems to inspect and review tens of layers of 3D NAND flash devices. To inspect and review hundreds of layers of 3D NAND flash devices, e-beam landing energies of up to about 50 keV to 100 keV may be required.

[0017] Secondly, conventional electron beam systems using BSE signals have a limited DOF of the primary electron beam. To increase the DOF, a smaller NA of the focused electron beam (i.e., the beam half angle β in FIG. 3) can be used. Systems with lower beam or landing energy have several drawbacks. With a smaller NA, optical blurring due to Coulomb interactions between the primary electrons becomes dominant. This is because a narrower electron beam results in a higher electron density. Towards the image side of the optical system, the electron gun lens aberrations become larger due to the smaller NA or larger optical magnification. Thus, the composite resolution at the wafer is reduced. These gun lens aberrations can dominate over the objective lens aberrations in low NA optical systems, especially when the gun lens is an electrostatic lens.

[0018] Figures 5(a) and 5(b) show how the use of a commercial electron beam system (beam voltage 10 kV, landing energy 9 keV) is limited by performing a Monte Carlo simulation of the system. The primary electrons are widely distributed with long tails in the x and y directions. Even for a large memory hole of 0.3 × 0.3 μm, the electrons can only be 95% (Fig. 5(a)) at the top of the hole and 97% (Fig. 5(b)) at the bottom (the “percentage shown” in each figure), and since the top of the hole is z = -3.5 μm and the bottom is z = 4.5 μm, the aspect ratio is only AR = 0.3:8.0 = 1:27. If the size of the hole is narrowed to 0.1 × 0.1 μm, the aspect ratio increases to AR = 1:81, but most of the outer electrons should be stopped at the top edge of the memory hole. These outer electrons generate BSEs and contaminate the BSE signal from the bottom of the memory hole.

[0019] The yield of BSE is usually much smaller than that of SE. To obtain a sufficient signal-to-noise ratio, the beam current of the primary electron beam may be increased. However, the Coulomb interactions between electrons are quite sensitive to this beam current, which can rapidly broaden the tail of the electron distribution and further contaminate the normal signal from the bottom of the memory hole. [Prior art documents] [Patent documents]

[0020] [Patent Document 1] US Patent Application Publication No. 2016 / 0163502 [Patent Document 2] US Patent Application Publication No. 2009 / 0179151 [Patent Document 3] International Publication No. 2014 / 142745 Summary of the Invention [Problem to be solved by the invention]

[0021] Therefore, there is a need for improved systems and methods for generating electron beams. [Means for solving the problem]

[0022] In a first embodiment, a system is provided, the system includes an electron beam source generating an electron beam, a beam limiting aperture disposed on an optical path of the electron beam, a lower Wehnelt electrode disposed on an optical path of the electron beam, an upper Wehnelt electrode disposed on an optical path of the electron beam between the lower Wehnelt electrode and the beam limiting aperture, an annular detector disposed on a surface of the upper Wehnelt electrode facing the lower Wehnelt electrode, a magnetic focusing lens disposed on an optical path of the electron beam between the upper Wehnelt electrode and the beam limiting aperture, a deflector disposed on an optical path of the electron beam between the upper Wehnelt electrode and the focusing lens, a magnetic objective lens coil disposed on an optical path of the electron beam between the deflector and the upper Wehnelt electrode, and a grounded tube disposed on an optical path of the electron beam. The magnetic focusing lens includes a pole piece and a focusing lens coil. The magnetic objective lens coil is disposed around the grounded tube.

[0023] The system may include an aperture disposed in an optical path of the electron beam between the focusing lens and the beam-limiting aperture.

[0024] The electron beam source may include a tip with a radius of less than 1 μm.

[0025] The deflector may be a magnetic deflector or a Wien filter. In one embodiment, the deflector is a magnetic deflector. The system may further include an upper magnetic deflector and an intermediate magnetic deflector. The upper magnetic deflector is disposed on the optical path of the electron beam between the deflector and the magnetic focusing lens. The intermediate magnetic deflector is disposed on the optical path of the electron beam between the upper magnetic deflector and the magnetic deflector.

[0026] The upper magnetic deflector may be configured to deflect the electron beam toward the intermediate magnetic deflector, which may be configured to deflect the electron beam toward the magnetic deflector in a direction opposite to a deflection direction of the upper magnetic deflector, which may be configured to deflect and collimate the electron beam along the optical path toward the grounded tube.

[0027] Each of the magnetic deflector, the upper magnetic deflector, and the middle magnetic deflector may have eight magnetic pole pieces that are rotationally symmetric.

[0028] The system may include a side detector disposed between the intermediate magnetic deflector and the magnetic deflector. The side detector may be configured to collect at least secondary electrons.

[0029] The system may further include an electron beam bender disposed between the intermediate magnetic deflector and the magnetic deflector, the electron beam bender may be configured to remove secondary electrons from backscattered electrons at the side detector.

[0030] In a second embodiment, a method is provided. The method includes generating an electron beam with an electron beam source. The electron beam is directed through a beam-limiting aperture. The electron beam is directed by a magnetic focusing lens disposed downstream of the beam-limiting aperture along its optical path. The electron beam is directed by a deflector disposed downstream of the magnetic focusing lens along its optical path. The electron beam is directed by an objective lens, the objective lens including a grounded tube, an upper Wehnelt electrode, and a lower Wehnelt electrode. The electron beam is directed from the lower Wehnelt electrode toward a surface of a wafer. Backscattered electrons from the wafer are received at an annular detector disposed on a surface of the upper Wehnelt electrode. The surface of the upper Wehnelt electrode faces the lower Wehnelt electrode.

[0031] In one example, the electron beam has a beam voltage of 50 kV to 100 kV and a landing energy of 50 keV to 100 keV.

[0032] The magnetic focusing lens may be configured to form the electron beam having a small depth of focus mode and a large depth of focus mode, the numerical aperture being smaller in the large depth of focus mode than in the small depth of focus mode.

[0033] The wafer may include a three-dimensional semiconductor structure.

[0034] In one example, the focal depth of the electron beam is up to 20 μm.

[0035] The electron beam source may include a tip with a radius of less than 1 μm.

[0036] The deflector may be a magnetic deflector or a Wien filter. In one example, the deflector is a magnetic deflector. The method may further include directing the electron beam through an upper magnetic deflector and an intermediate magnetic deflector disposed along the optical path between the deflector and the magnetic focusing lens.

[0037] The upper magnetic deflector may be configured to deflect the electron beam toward the intermediate magnetic deflector, which may be configured to deflect the electron beam toward the magnetic deflector in a direction opposite to a deflection direction of the upper magnetic deflector, which may be configured to deflect and collimate the electron beam along the optical path toward the grounded tube.

[0038] The method can include receiving secondary electrons at a side detector disposed between the intermediate magnetic deflector and the magnetic deflector.

[0039] The method may include bending electrons returning from the wafer between the intermediate magnetic deflector and the magnetic deflector, thereby filtering out secondary electrons from backscattered electrons at the side detector.

[0040] For a fuller understanding of the nature and objects of the present disclosure, reference should be made to the following detailed description taken in conjunction with the accompanying drawings. [Brief description of the drawings]

[0041] [Figure 1] 1 is a schematic diagram of an exemplary 3D NAND flash memory. [Diagram 2] FIG. 2 is a model diagram of stacked bits and memory holes in an example 3D NAND flash device. [Diagram 3] FIG. 1 is a model diagram of a profile of a focused electron beam in an exemplary HAR memory hole. [Figure 4] 1 is a schematic diagram of a BSE penetrating a material from the bottom of an exemplary memory hole and exiting the top surface of the material. [Figure 5(a)] This is a distribution diagram of primary electrons at the top (z=-3.5 μm) of a 0.3×0.3 μm memory hole. [Figure 5(b)] This is a distribution diagram of primary electrons at the bottom (z=4.5 μm) of a 0.3×0.3 μm memory hole. [Figure 6(a)]FIG. 1 illustrates a first embodiment of an electron beam system. [Figure 6(b)] FIG. 7 shows the optical system of a first embodiment of the electron beam system of FIG. 6(a). [Figure 7(a)] This is a distribution diagram of primary electrons at the top (z=-10 μm) of a 0.1×0.1 μm memory hole. [Figure 7(b)] This is a distribution diagram of primary electrons at the bottom (z=10 μm) of a 0.1×0.1 μm memory hole. [Figure 8] FIG. 2 shows a second embodiment of a system for detecting BSE. [Figure 9] FIG. 13 shows a third embodiment of the electron beam separation optical system with three magnetic deflectors. [Figure 10] FIG. 10 is a diagram showing an eight-pole magnetic field deflector in the third embodiment of FIG. [Figure 11] 10 is a diagram showing isopotential lines in the eight-pole magnetic field deflector of the third embodiment of FIG. 9. FIG. [Figure 12] FIG. 10 illustrates collection of SE / BSE in the third embodiment of FIG. 9 for use with low landing energy. [Figure 13] FIG. 13 shows ray tracing simulation of SE / BSE for low landing energy utilization. [Figure 14] FIG. 1 illustrates one embodiment of an electron beam bender. [Figure 15] 1 is a flow chart of a method according to the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0042] Although the claimed subject matter is described with respect to certain embodiments, other embodiments, including embodiments that do not provide all of the advantages and features described herein, are also within the scope of the disclosure. Various structural, logical, process step, and electronic component changes can be made without departing from the scope of the disclosure. Accordingly, the scope of the disclosure is defined solely by reference to the appended claims.

[0043] As the 2D planar processes of nanotechnology become increasingly flawed, the development of 3D nanofabrication processes for chips is progressing. Many chip devices, such as 3D NAND flash, 3D DRAM, and 3D logic, are constructed to contain memory holes, channel holes, staircase steps, and deep trenches that may require inspection and review during manufacturing. For example, a wafer for a 96-layer 3D NAND flash contains hundreds of billions of memory holes that are tens of microns deep.

[0044] The embodiments disclosed herein can provide a DOF of tens of microns for high AR structures (e.g., AR=1:100 or greater) of 3D chip devices. These embodiments can provide a DOF of 20 μm with a 60 kV e-beam system, which is compatible with the inspection and review of wafers with AR=1:200. These embodiments can also provide a high resolution system for inspecting and reviewing the surface of a wafer, which simultaneously images SE and BSE.

[0045] To address the shortcomings of conventional systems, an electron beam system may be configured to provide high resolution, short tail of electron distribution, and large DOF for inspecting and reviewing high AR electronic devices. The beam voltage (BV) may be increased to about 50 kV to 100 kV, which may be adjustable. This voltage may allow electron landing energy of about 50 keV to 100 keV. This mechanical design may avoid high voltage (HV) arcing. Given a pair of beam voltage and landing energy, the beam current may be increased, thereby increasing the BSE production and increasing throughput and signal to noise ratio. The maximum beam current may be selected by limiting the outer electrons that strike both the top and bottom edges of the memory hole to, for example, less than 2% of the total primary electrons.

[0046] FIG. 6(a) shows a first embodiment of an electron beam system 100. FIG. 6(b) shows a schematic diagram of the optical system of the embodiment of FIG. 6(a). In the electron beam system 100, a magnetic gun lens may be used. The use of the magnetic gun lens can reduce the aberration of the gun lens. Therefore, even if the optical magnification is high when the NA is small or the DOF is large, there is almost no blurring effect from the gun lens on the wafer side. The use of the magnetic gun lens may also enable focusing and defocusing of the high voltage electron beam to select the desired beam current.

[0047] The electron beam system 100 includes an electron beam source 101 that generates an electron beam 103. The electron beam 103 is directed toward a wafer 110. The wafer 110 may be disposed on a platen. The electron beam source 101 may include a tip with a radius of less than 1 μm, for example, about 0.3 μm to less than 1.0 μm. The electron beam source 101 may also include an extractor 115, an anode 116, a gun field lens coil 117, and a gun field lens pole piece 118. These components may be part of a gun lens (GL).

[0048] The small tip radius of a thermal field emission (TFE) source (e.g., low submicron range) can provide high brightness and a small virtual source. For optical systems with high optical magnification (e.g., about 1.0X) and low NA, the size of the virtual source can affect the degradation of the resolution. Chromatic aberration may be less significant for optical systems with high BV and high LE, but the spread of the source energy can be large due to the small tip radius of a TFE source.

[0049] A beam limiting aperture (BLA) 102 is disposed in the optical path of the electron beam 103. The beam limiting aperture 102 may select a maximum beam current for a particular application. The beam limiting aperture 102 may include an aperture having a diameter or cross-sectional length of about 50 μm to about 100 μm for various beam currents.

[0050] Aperture 104 may be disposed on the optical path of electron beam 103 between focusing lens 105 and beam limiting aperture 102. Aperture 104 may have a diameter or cross-sectional length of about 10 μm to about 30 μm. The position of the crossover (xo) may be adjustable based on the strength of the gun lens. Aperture 104 may be a lens barrel aperture.

[0051] A magnetic focusing lens 105 is disposed in the path of the electron beam 103 between the upper Wehnelt electrode 107 and the beam limiting aperture 102. For example, the magnetic focusing lens 105 may be disposed in the path of the electron beam 103 between the aperture 104 and the deflector 106. The magnetic focusing lens 105 may include a pole piece 111, a focusing lens coil 112, and a shield 125.

[0052] The magnetic focusing lens 105 allows the selection of an optimum NA or a smaller NA for a larger DOF. An electrostatic focusing lens may not be strong enough to focus a high energy electron beam because the focusing voltage may be limited to avoid arcing problems. The magnetic focusing lens 105 can focus the beam further. The magnetic focusing lens 105 includes a pole piece 111 and a coil 112.

[0053] A deflector 106 is disposed in the optical path of the electron beam 103 between the upper Wehnelt electrode 107 and the focusing lens 105. The deflector 106 can be a magnetic deflector or a Wien filter.

[0054] An objective lens (OL) 119 may be included. A magnetic objective lens coil 113 is disposed in the optical path of the electron beam 103 between the deflector 106 and the upper Wehnelt electrode 107. A grounded tube 114 is disposed in the optical path of the electron beam 103 such that the magnetic objective lens coil 113 is disposed around the grounded tube 114. The grounded tube 114 may face the electron beam 103. Thus, the grounded tube 114 is between the electron beam 103 and the magnetic objective lens coil 113.

[0055] The objective lens 119 is a compound lens including an electrostatic objective lens and a magnetic objective lens. The objective lens 119 includes a grounded tube 114, an upper Wehnelt (UW) electrode 107, and a lower Wehnelt (LW) electrode 108. The upper Wehnelt electrode 107 and / or the lower Wehnelt electrode 108 may be electrostatic. The objective lens 119 may have a suitable working distance that minimizes lens aberrations without saturating the objective lens pole piece 120. In one example, the working distance is about 1 mm to 3 mm. When the landing energy is low (e.g., about 0.1 to 10 keV), a working distance of about 1 mm may be used. When the LE is high (e.g., about 30 to 60 keV), a working distance of about 3 mm may be used. The objective lens 119 may also include a pole piece 120 and a coil 113. In one example, the lower pole piece may be used as the lower Wehnelt (LW) electrode 108.

[0056] An upper Wehnelt electrode 107 is disposed in the path of the electron beam 103 between the lower Wehnelt electrode 108 and the beam limiting aperture 102. For example, the upper Wehnelt electrode 107 may be disposed in the path of the electron beam 103 between the deflector 106 and the lower Wehnelt electrode 108. The upper Wehnelt electrode 107 may be grounded, negatively floating, or positively floating depending on various applications.

[0057] The top Wehnelt electrode 107 may be operated with a positive voltage. This may be to reduce the chromatic aberration of the objective lens, to reduce the effect of Coulomb interaction, or to aid in the collection of BSEs. The shape of the top Wehnelt electrode 107 may be configured to accommodate the annular detector 109. The annular detector 109 is used to increase the collection efficiency of the BSEs. The central aperture size of the top Wehnelt electrode 107 may be about 1 mm (as shown in FIG. 12). This allows all SEs and low-angle BSEs to pass through and be collected by the side detector 123.

[0058] A lower Wehnelt electrode 108 is disposed on the optical path of the electron beam 103. The lower Wehnelt electrode 108 can apply an extraction electric field to the wafer 110 suitable for various inspections of the wafer layer. The extraction electric field can be about 0 V / mm to 2000 V / mm. A low extraction electric field (e.g., about 0 to 500 V / mm) is typically used for inspection, and a high extraction electric field (e.g., about 1000 to 2000 V / mm) is typically used for review.

[0059] An annular detector 109 is disposed on the surface of the upper Wehnelt electrode 107 facing the lower Wehnelt electrode 108. The annular detector 109 is a detection device capable of collecting BSE. The annular detector 109 centered on the optical axis can collect BSE generated at the bottom of the memory hole and penetrating the stacked thin film material. The annular detector 109 (e.g., a semiconductor detector) centered on the optical axis may be embedded at the bottom of the upper Wehnelt electrode 107. The active area of ​​the annular detector 109 may face the wafer 110, and its backside may generally be biased to several hundred volts (e.g., about 100 to 300 V) for forming a detection signal circuit.

[0060] The tip radius of the electron beam source 101 may be in the low submicron range. The voltage of the extractor 115 may be set higher than the tip bias to obtain sufficient brightness and angular intensity of the source. The electrons emitted from the tip are accelerated by the anode 116 to about 50 kV to 100 kV and focused by the magnetic focusing lens 105 to form an electron beam crossover between the beam limiting aperture 102 and the aperture 104. The aperture 104 may select the beam current that actually reaches the wafer 110. In FIG. 6(a), a Wien filter or a magnetic deflector may be placed to separate SEs and / or BSEs from primary electrons (PEs). The aperture 104 may be an independent aperture rod containing multiple apertures. If one aperture is damaged or contaminated, another aperture may be moved to replace the damaged or contaminated aperture.

[0061] In FIG. 6(b), there may be two imaging modes for different applications. One imaging mode is a high resolution (RES) mode with a relatively small DOF. The other imaging mode is a large DOF mode with a relatively low resolution. In the high resolution mode, the focusing lens 105 focuses the electron beam 103 to form an optimal NA (e.g., NA2 in FIG. 6(b)). At this optimal NA, the electron beam spot size on the wafer is smallest. In the large DOF mode, the focusing lens 105 further focuses the beam to form a smaller NA (e.g., NA1 in FIG. 6(b)). At this NA, the DOF is maximized, thus meeting the requirements of inspection and review of high AR devices such as 3D NAND, 3D DRAM, and deep trench logic.

[0062] Figures 7(a) and 7(b) show the results of a Monte Carlo simulation of the electron optical column of Figure 6(a) in the large DOF mode, where the beam energy is 60 kV and the landing energy is 55 keV. In this simulation, the upper Wehnelt electrode is grounded. In the simulations of Figures 7(a) and 7(b), the beam current and extraction field at the wafer are set to be the same as in Figures 5(a) and 5(b) for comparison with Figures 5(a) and 5(b).

[0063] In Figures 7(a) and 7(b), the primary electrons are distributed in a narrow range with short tails in the x and y directions. This is due to the reduced Coulomb interaction between electrons with high beam voltage. As shown in Figures 7(a) and 7(b), even though the memory hole size is small at 0.1 × 0.1 μm, the electrons contained are more than 98% (indicated percentages in each figure) at the top (Figure 7(a)) and bottom (Figure 7(b)) of the hole, respectively, and the aspect ratio is high, AR = 0.1:20 = 1:200, since the top of the hole is z = -10 μm and the bottom is z = 10 μm. The DOF of 20 μm in this application is large. Nevertheless, the resolution is better than that of Figures 5(a) and 5(b) when comparing the spot sizes measured in Figures 7(a) and 7(b) at current amounts of 20-80% and 12-88% with those in Figures 5(a) and 5(b).

[0064] When electrons with high landing energy (e.g., about 50 keV to 100 keV) strike the bottom of the memory hole, they generate the same high-energy BSEs that penetrate and exit through the stack of materials consisting of hundreds of thin films of 3D NAND layers as shown in Figure 4. These BSEs can be collected by a BSE detector located inside the primary electron beam column.

[0065] 8 is a second embodiment of a system for detecting BSE. An annular detector 109 centered on the optical axis for detecting BSE may be at least partially inside the upper Wehnelt electrode 107. In one example, a surface of the annular detector 109 may be exposed to BSE. In another example, the annular detector 109 is completely inside the upper Wehnelt electrode 107 and is not exposed to BSE, but still allows for BSE collection.

[0066] The top Wehnelt electrode 107 may be floating or grounded. The bottom Wehnelt electrode 108 may be used to apply the required extraction field to the wafer. Using the same conditions as in Figures 7(a) and 7(b), simulations of ray tracing of BSEs from the bottom of the memory hole of a 3D NAND show high collection efficiency. Thus, BSEs with polar angles of about 10 degrees to 60 degrees can be detected. Due to the angular distribution characteristics of BSEs, most of the total amount of emitted BSEs is contained in angles of about 10 to 60 degrees.

[0067] 9 is a third embodiment of the electron beam separation optical system with three magnetic deflectors. In the embodiment of FIG. 9, the deflector 106 is a magnetic deflector (MD-3). If we only collect and use high-energy BSEs to inspect high AR devices such as 3D NAND memory, we do not need optical designs such as the conventional Wien filter to separate BSEs from PEs (primary electrons).

[0068] The system also includes an upper magnetic deflector 121 (MD-1) and an intermediate magnetic deflector 122 (MD-2), which are disposed on the optical path of the electron beam 103 between the deflector 106 and the magnetic focusing lens 105. The upper magnetic deflector 121 is disposed on the optical path of the electron beam 103 between the deflector 106 and the magnetic focusing lens 105. The intermediate magnetic deflector 122 is disposed on the optical path of the electron beam 103 between the upper magnetic deflector 121 and the magnetic deflector 106.

[0069] The upper magnetic deflector 121 may be configured to deflect the electron beam 103 towards the intermediate magnetic deflector 121. The intermediate magnetic deflector may be configured to deflect the electron beam 103 in a direction opposite to the deflection direction of the upper magnetic deflector 121 towards the magnetic deflector 106. The magnetic deflector is configured to deflect and collimate the electron beam 103 along an optical path towards the grounded tube 114.

[0070] There can be two optical modes of operation for the embodiment of the configuration of FIG. 6. One mode is a large DOF mode with a small NA for inspecting and reviewing HAR NAND flash memory. The other mode is a high resolution mode with an optimal NA for inspecting and reviewing physical defects on the wafer surface or voltage contrast defects tens to hundreds of nanometers below the wafer surface. In the high resolution mode, all lens aberrations and all blurring caused by Coulomb interactions between electrons are balanced. The landing energy (LE) for inspecting the wafer surface is required to be low within hundreds to thousands of electron volts. SEs can be the signals collected for inspection and review. Because of their low emission energy (e.g., a few electron volts), SEs can be strongly focused by the magnetic field of the objective lens. Therefore, SEs are not necessarily well collected by the annular detector, but can be collected by the side detector instead.

[0071] To separate the SE from the PE, a Wien filter is used as usual. However, the Wien filter may cause a color blur due to the spread of the light source energy. To remove this color blur, another Wien filter may be used to compensate for the trajectory deviation caused by the spread of the light source energy. This may achieve the desired behavior, but may increase the complexity of the optical system.

[0072] As shown in Fig. 9, an electron beam separation optical system without a Wien filter can be used. Three magnetic deflectors (upper magnetic deflector 121, middle magnetic deflector 122, and deflector 106) are arranged between the focusing lens 105 and the objective lens 119. The upper magnetic deflector 121 deflects the electron beam 103 from the source to the middle magnetic deflector 122, which deflects the electron beam 103 in the opposite direction to the deflector 106, which deflects the electron beam 103 back again and collimates it along the optical axis to the objective lens 119. The deflection angle can be related to the side detector 123. In Fig. 12 or Fig. 14, since there is a side detector 123 in the xoz plane, the primary electrons are deflected in the opposite direction to the same xoz plane. For this xoz deflection of the primary electrons, the magnetic flux field of the magnetic deflector may be in the yoz plane, i.e. the rotation angle of the magnetic deflector according to FIG. 11 is fixed.

[0073] Each of the top magnetic deflector 121, the middle magnetic deflector 122, and the deflector 106 may have the same structure and / or configuration, such as those shown in FIG. 10 and FIG. 11. FIG. 10 is a cross-sectional view of the magnetic deflector. Eight pole pieces are arranged rotationally symmetrically as an octupole deflector. The same number of coil turns ("N" in FIG. 11) are wrapped around each pole piece. The pole pieces are shielded as shown. By appropriately setting the current through the coils, the distribution of the deflection magnetic field can be fairly uniform over a large area. Thus, coma due to large angle deflection is minimized. FIG. 11 shows a simulation of a uniform deflection magnetic field with coil currents applied as Ix=1 unit, Ixy=1 / √2 unit, and Iy=0 unit using magnetic isopotential lines. As shown in FIG. 9, the magnetic flux of the magnetic field in the x-axis deflects the primary electron beam in the y-axis.

[0074] Even with low landing energy, the embodiment disclosed herein can increase the imaging resolution of the primary electron beam because the large and optimal NA (NA2 in FIG. 6(b)) reduces the Coulomb interaction between electrons. Half of the total spot size can be caused by the objective lens aberration and the other half by the Coulomb interaction. The effect of the objective lens aberration and the Coulomb interaction can be further reduced by using the upper Wehnelt electrode 107 as disclosed in FIG. 9. The upper Wehnelt electrode 107 can be considered as an energy intensifier. The voltage applied to the upper Wehnelt electrode 107 can be higher than the beam voltage. If the same extraction field is applied to the wafer 110 through the lower Wehnelt electrode 108, simulations show that the voltage of the upper Wehnelt electrode 107 higher than the beam voltage leads to smaller spherical and chromatic aberration coefficients of the objective lens 119 and simultaneously lowers the Coulomb interaction effect.

[0075] In the three magnetic deflector embodiment of the configuration of FIG. 9, the forward color blur that usually occurs in the Wien filter due to the spread of the source energy can be eliminated. In the Wien filter-based optical system, one more additional Wien filter can be used to compensate for the trajectory deviation due to the spread of the source energy. In the three magnetic deflector configuration of FIG. 9, the net total magnetic deflection angle is zero, which makes the sum of the electron trajectory deviation due to all of the source energy spread, electronic noise, and / or thermal noise zero.

[0076] Figure 12 shows the collection of SE / BSE in the third embodiment of Figure 9 for use with low landing energy. Figure 13 shows a ray tracing simulation of SE / BSE for low landing energy. A side detector 123 is disposed between the intermediate magnetic deflector 122 and the magnetic deflector 106. Typically, the deflection angle θ of the SE from the deflector 106 is about 6°-12°, i.e., the distance from the deflector 106 to the side detector 123 is about 100 mm or more.

[0077] The side detector 123 is configured to collect at least the SEs. The side detector 123 is shown between the intermediate magnetic deflector 122 and the magnetic deflector 106, but may be between the upper magnetic deflector 121 and the intermediate magnetic deflector 122. The side detector 123 may also be upstream of the upper magnetic deflector 121. The side detector 123 may be a semiconductor detector, a fast scintillator detector, or other system.

[0078] 12 and 13 show the collection of SE / BSE. Since the moving directions of PE and SE are opposite, the deflection angle of the deflector 106 of the SE, shown as angle θ in FIG. 12, is opposite to that of the PE. The side detector 123 is arranged below the intermediate magnetic deflector 122 in the embodiment of FIG. 12. The selection of the excitation of the deflector 106 can be made such that, for a given beam voltage and landing energy, it is possible to deflect the SE at an angle θ and guide it towards the center of the side detector 123. Taking into account the excitation of this deflector 106, the excitation of the upper magnetic deflector 121 and the intermediate magnetic deflector 122 can be adjusted to collimate the beam emitted from the source with respect to the optical axis of the objective lens.

[0079] At high resolution, used with low landing energies of hundreds to thousands of electron volts, low energy BSEs can also be collected in the manner shown in FIG. 13. All SEs have low emission energies (a few eV) and are immediately focused into a narrow beam by the magnetic field of the objective lens, as shown by the central beam in FIG. 13. The SEs are deflected by the deflector 106 and collected by the side detector 123. The BSEs have higher emission energies (same energy as the landing energy) and can be collected by both the side detector 123 and the annular detector 109. BSEs with small polar angles (e.g., less than 10 degrees) can be focused and accelerated through a hole in the upper Wehnelt electrode 107, as shown by the ray of BSE-1 in FIG. 13. Electrons of BSE-1 can also be collected by the side detector 123 to form a bright-field image. BSEs with large polar angles (e.g., greater than 45 degrees) can be collected by the annular detector 109 to form a dark-field image, as shown by the ray of BSE-2 in FIG. 13.

[0080] 14 is one embodiment of the electron beam bender 124. The electron beam bender 124 is disposed between the intermediate magnetic deflector 122 and the magnetic deflector 106, or at another location proximate to the side detector 123. The electron beam bender 124 has two cylindrical surfaces and is configured to remove SEs from the BSEs at the side detector 123. Thus, the electron beam bender 124 can be used to remove SEs from the BSEs, or vice versa.

[0081] The SE or BSE may be bent depending on where the side detector 123 is located. For example, in FIG. 14, the beam is bent 90 degrees (about 10 degrees by deflection of the deflector 106, and the rest by the electron beam bender 124). The bending voltage may be independent of the bending angle, so the same voltage may be used for any bending angle. The bending voltage may depend on the energy of the SE / BSE and the gap of the electron beam bender 124. The smaller the gap, the lower the bending voltage. The gap may be in the range of about 8 mm to 16 mm. For applications where the landing energy is low and SEs are to be collected, the bending voltage may be 2 to 3 kV.

[0082] In FIG. 12, due to the use of low landing energy, the energy difference between the SE and BSE-1 beams is narrow, only a few hundred to a few thousand electron volts. SE and BSE-1 electrons can be collected together at the side detector 123, thereby contaminating each other's imaging signals. With the electron bender 124 of FIG. 14, the SE beam can be removed from the BSE-1 beam by a suitable bending voltage Vb. The SE beam is deflected more by the bender since it has lower energy, and the BSE-1 electrons pass through the exit aperture towards the side detector 123. The SE can also be selected from the BSE-1 beam. The selection is done by deflecting the BSE-1 beam less and stopping it at the cylindrical surface of the electron beam bender 124 by using a lower bending voltage Vb.

[0083] In addition to the benefit of filtering BSEs from SEs (or vice versa), embodiments that include the electron beam bender 124 may provide a space between the exit of the electron beam bender 124 and the side detector 123. Additional energy filtering of the SEs or BSEs may be performed by configuring a low pass, high pass, or band filter in this space. This may be done before directing either the SEs or BSEs towards the side detector 123, allowing for more careful analysis and learning cycles of the defect characteristics.

[0084] When the SE / BSE is bent at 90 degrees, the side detector 123 can be placed at a certain distance away from the exit of the electron beam bender 124. Signal processing optics can be placed between the exit of the electron beam bender 124 and the side detector 123. The signal processing optics can be a low pass energy filter, a high pass energy filter, a band pass energy filter, a descan deflector, or other systems for evaluating more detailed wafer defect characteristics. Without the electron beam bender 124 as in FIG. 12, the SE / BSE cannot be deflected at a large angle, and the side detector 123 cannot be placed to accommodate such signal processing optics.

[0085] The electron beam bender 124 of Figure 14 may be electrostatic or magnetic. If the bending of the electrons is in one plane, then magnetic flux may be oriented in the perpendicular plane.

[0086] FIG. 15 is a flow chart of method 200. Method 200 may be performed using one embodiment of the electron beam system disclosed herein. An electron beam is generated at 201 and directed through a beam limiting aperture at 202. The electron beam may have a beam voltage of 50 kV to 100 kV and a landing energy of 50 keV to 100 keV. The choice of beam or landing energy depends on the application. The beam energy is usually equal to or greater than the landing energy. For deep memory holes with many layers of thin films, a higher landing energy may be used to obtain a high BSE energy. This is to allow the BSE to penetrate the thickly stacked thin films and escape from the surface. 50 keV may be for about 96 to 124 layers. 100 keV may be for more than 256 layers.

[0087] At 203, the electron beam is directed through a magnetic focusing lens disposed downstream of the beam limiting aperture along its optical path. At 204, the electron beam is directed through a deflector disposed downstream of the magnetic focusing lens along its optical path. The deflector may be a magnetic deflector or a Wien filter. At 205, the electron beam is directed through an objective lens. The objective lens includes a grounded tube, an upper Wehnelt electrode, and a lower Wehnelt electrode. At 206, the electron beam is directed from the lower Wehnelt electrode toward a surface of a wafer. The focal depth of the electron beam may be up to 20 μm. The wafer may include a three-dimensional semiconductor structure such as 3D NAND, 3D DRAM, 3D staircase stage, or 3D deep trench logic. At 207, backscattered electrons from the wafer are received at an annular detector disposed on a surface of the upper Wehnelt electrode facing the lower Wehnelt electrode.

[0088] The magnetic focusing lens may be configured to form an electron beam having a small depth of focus mode and a large depth of focus mode, the numerical aperture being smaller in the large depth of focus mode than in the small depth of focus mode.

[0089] In one example, the deflector is a magnetic deflector. The electron beam may be further directed along its optical path through an upper magnetic deflector and an intermediate magnetic deflector disposed between the deflector and the magnetic focusing lens. The upper magnetic deflector may be configured to deflect the electron beam toward the intermediate magnetic deflector. The intermediate magnetic deflector may be configured to deflect the electron beam toward the magnetic deflector in a direction opposite to the deflection direction of the upper magnetic deflector. The magnetic deflector may be configured to deflect and collimate the electron beam along the optical path to a grounded tube.

[0090] Secondary electrons can be received at a side detector disposed between the intermediate magnetic deflector and the magnetic deflector. Electrons returning from the wafer can be bent between the intermediate magnetic deflector and the magnetic deflector, thereby separating the secondary electrons from the backscattered electrons at the side detector. Bending the electron beam can separate BSE from SE (or vice versa), thereby removing electronic contamination from the useful signal.

[0091] Each embodiment of the system and method described herein can be used for inspection and review of deep memory holes. For example, operation at 60 kV provides a DOF of 20 μm for AR=1:200 applications, which is more than 5 times better than conventional designs. Using a high BV and high LE, the BSEs are less focused compared to conventional relatively low LE BSE collection, so a high BSE collection efficiency can be achieved using an annular detector. With the advantage of a sufficient BSE signal, a relatively low beam current can be used. This further reduces the Coulomb interaction between electrons, greatly extending the DOF while simultaneously shortening the tail of the electron distribution.

[0092] The Wien-filter-free electron beam can be separated by an optical system with three magnetic deflectors, which can eliminate the transmission chromatic aberration due to the spread of the source energy and electronic and / or thermal noise when the net total deflection angle is zero.

[0093] An upper Wehnelt electrode can be used to boost (i.e. accelerate) the beam energy before it is decelerated towards the wafer, thereby reducing the spherical and chromatic aberration coefficients of the objective lens. The boost in beam energy simultaneously reduces the Coulomb interaction between the electrons, which improves the resolution for applications with low landing energy.

[0094] Method 200 can be used for inspection and review of 3D devices (e.g., 3D NAND, 3D DRAM, 3D staircase steps, and 3D deep trench logic). The embodiment of Figures 6 and 8 provides a large DOF of up to 20 microns for inspection and review of 3D devices with AR=1:200. This can be used for development of hundreds of layers of next generation 3D NAND memory.

[0095] To inspect and review the wafer surface or low sub-micron surface defects with low landing energy of hundreds to thousands of electron volts, the embodiments disclosed herein can use the Wien filter-free beam separation optics with three magnetic deflectors of Figures 9-11, and the upper Wehnelt electrode for energy enhancement can reduce the spherical and chromatic aberration coefficients of the objective lens (Figures 12 and 13). The electron beam bender can remove the BSE from the SE or the SE from the BSE (Figure 14). Thus, high resolution imaging and high collection efficiency of SE and BSE can be achieved simultaneously.

[0096] Although described with respect to an electron beam, the embodiments disclosed herein may also be used with ion or particle beams.

[0097] Although the present disclosure has been described with respect to one or more particular embodiments, it will be understood that other embodiments of the present disclosure may be constructed without departing from the scope of the present disclosure. Accordingly, the present disclosure is deemed to be limited only by the appended claims and their reasonable interpretation.

Claims

1. an electron beam source for generating an electron beam; a beam limiting aperture disposed on an optical path of the electron beam; a lower electrode disposed on an optical path of the electron beam; an upper electrode disposed on an optical path of the electron beam between the lower electrode and the beam limiting aperture; an annular detector disposed on a surface of the upper electrode facing the lower electrode; a magnetic focusing lens disposed on an optical path of the electron beam between the upper electrode and the beam limiting aperture, the magnetic focusing lens including a pole piece and a focusing lens coil; a deflector disposed on an optical path of the electron beam between the upper electrode and the focusing lens; a magnetic field objective lens coil disposed on an optical path of the electron beam between the deflector and the upper electrode; a grounded tube disposed on an optical path of the electron beam, the grounded tube having the magnetic field objective lens coil disposed therearound; Including, A system having two modes, a high resolution (RES) mode in which the depth of focus is relatively small, and a low resolution mode in which the depth of focus is relatively large, wherein in the high resolution mode, the magnetic focusing lens focuses the electron beam to form an optimal numerical aperture, and in the low resolution mode, the magnetic focusing lens focuses the electron beam to form a smaller numerical aperture to perform inspection and review of high AR devices including at least one of 3D NAND, 3D DRAM, and deep trench logic.

2. The system of claim 1 , further comprising an aperture disposed in an optical path of the electron beam between the focusing lens and the beam-limiting aperture.

3. The system of claim 1 , wherein the electron beam source includes a tip having a radius of less than 1 μm.

4. The system of claim 1 , wherein the deflector is a magnetic deflector or a Wien filter.

5. 5. The system of claim 4, wherein the deflector is a magnetic deflector, further comprising an upper magnetic deflector and an intermediate magnetic deflector, the upper magnetic deflector being disposed on the optical path of the electron beam between the deflector and the magnetic focusing lens, and the intermediate magnetic deflector being disposed on the optical path of the electron beam between the upper magnetic deflector and the magnetic deflector.

6. 6. The system of claim 5, wherein the upper magnetic deflector is configured to deflect the electron beam toward the intermediate magnetic deflector, the intermediate magnetic deflector is configured to deflect the electron beam toward the magnetic deflector in a direction opposite to a deflection direction of the upper magnetic deflector, and the magnetic deflector is configured to deflect and collimate the electron beam along the optical path toward the grounded tube.

7. The system of claim 5 , wherein the magnetic deflector, the upper magnetic deflector, and the middle magnetic deflector each have eight pole pieces that are rotationally symmetric.

8. The system of claim 5 , further comprising a side detector disposed between the intermediate magnetic deflector and the magnetic deflector, the side detector configured to collect at least secondary electrons.

9. 10. The system of claim 8, further comprising an electron beam bender disposed between the intermediate magnetic deflector and the magnetic deflector, the electron beam bender configured to remove secondary electrons from backscattered electrons at the side detector.

10. generating an electron beam with an electron beam source; directing the electron beam through a beam-limiting aperture; directing the electron beam through a magnetic focusing lens disposed along its optical path downstream of the beam-limiting aperture; directing the electron beam along its optical path through a deflector disposed downstream of the magnetic focusing lens; directing the electron beam through an objective lens, the objective lens including a grounded tube, an upper electrode, and a lower electrode; directing the electron beam from the bottom electrode towards a surface of a wafer; receiving backscattered electrons from the wafer at an annular detector disposed on a surface of the upper electrode, the surface of the upper electrode facing the lower electrode; Including, A method for performing inspection and review of high AR devices including at least one of 3D NAND, 3D DRAM, and deep trench logic, the method having two modes, a high resolution (RES) mode with a relatively small depth of focus, and a low resolution mode with a relatively large depth of focus, wherein in the high resolution mode, the magnetic focusing lens focuses the electron beam to form an optimal numerical aperture, and in the low resolution mode, the magnetic focusing lens focuses the electron beam to form a smaller numerical aperture.

11. The method of claim 10, wherein the electron beam has a beam voltage of 50 kV to 100 kV and a landing energy of 50 keV to 100 keV.

12. The method of claim 10 , wherein the wafer comprises a three-dimensional semiconductor structure.

13. The method of claim 10, wherein the electron beam has a focal depth of up to 20 μm.

14. The method of claim 10 , wherein the electron beam source comprises a tip with a radius of less than 1 μm.

15. The method of claim 10, wherein the deflector is a magnetic deflector or a Wien filter.

16. 16. The method of claim 15, wherein the deflector is a magnetic deflector, and further comprising directing the electron beam through an upper magnetic deflector and an intermediate magnetic deflector disposed along its optical path between the deflector and the magnetic focusing lens.

17. 17. The method of claim 16, wherein the upper magnetic deflector is configured to deflect the electron beam toward the intermediate magnetic deflector, the intermediate magnetic deflector is configured to deflect the electron beam toward the magnetic deflector in a direction opposite to a deflection direction of the upper magnetic deflector, and the magnetic deflector is configured to deflect and collimate the electron beam along the optical path toward the grounded tube.

18. The method of claim 16 , further comprising receiving secondary electrons at a side detector disposed between the intermediate magnetic deflector and the magnetic deflector.

19. 20. The method of claim 18, further comprising bending electrons returning from the wafer between the intermediate magnetic deflector and the magnetic deflector, thereby removing the secondary electrons from backscattered electrons at the side detector.

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