High-resolution electron beam equipment with dual aperture scheme

Abstract

An electron source emits an electron beam. The electron beam is received by a beam limiting assembly. The beam limiting assembly has a first beam limiting aperture with a first diameter and a second beam limiting aperture with a second diameter greater than the first diameter. The first beam limiting aperture receives the electron beam. The beam limiting assembly reduces the influence of Coulomb interaction.

Description

Technical Field

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

[0002] The evolution of the semiconductor manufacturing industry places higher demands on yield management and especially on metrology and inspection systems. Critical dimensions continue to shrink, and the industry needs to reduce the time used to achieve high-yield, high-value production. Minimizing the total time from detecting a yield problem to resolving the problem determines the return on investment for semiconductor manufacturers.

[0003] The manufacture of semiconductor devices such as logic and memory devices typically includes processing a semiconductor wafer using a number of manufacturing processes to form the various features and multiple levels of the semiconductor devices. For example, photolithography is a semiconductor manufacturing process that involves transferring a pattern from a reticle to a photoresist arranged on a semiconductor wafer. Additional examples of semiconductor manufacturing processes include, but are not limited to, chemical mechanical polishing (CMP), etching, deposition, and ion implantation. An arrangement of multiple semiconductor devices manufactured on a single semiconductor wafer may be separated into individual semiconductor devices.

[0004] Inspection processes are used at various steps during semiconductor manufacturing to detect defects on wafers to promote higher yields and therefore higher profits in the manufacturing process. Inspection has always been an important part of manufacturing semiconductor devices, such as integrated circuits (ICs). However, as the size of semiconductor devices decreases, inspection becomes even more important for the successful manufacture of acceptable semiconductor devices because smaller defects can cause device failures. For example, as the size of semiconductor devices decreases, the detection of defects with reduced sizes has become necessary because even relatively small defects can cause undesirable aberrations in semiconductor devices.

[0005] However, as design rules shrink, semiconductor manufacturing processes can operate closer to the limits of the performance capabilities of the process. In addition, as design rules shrink, smaller defects can have an impact on the electrical parameters of the device, which drives more sensitive inspection. As design rules shrink, the population of potential yield-related defects detected by inspection grows significantly, and the population of disturbing point defects detected by inspection also increases significantly. Therefore, more defects can be detected on the wafer, and it can be difficult and expensive to correct the process to eliminate all defects. Determining which defects actually have an impact on the electrical parameters and yield of the device allows process control methods to focus on the defects and generally ignore other defects. In addition, under smaller design rules, in some cases, process-induced failures tend to be systematic. In addition, process-induced failures tend to fail under predetermined design patterns that are usually repeated many times within the design. The elimination of spatially systematic, electrically related defects can have an impact on yield.

[0006] Focused electron beam systems (e-beam) are commonly used to create or inspect microstructures of items such as silicon wafers used in the manufacture of integrated circuits. The electron beam is formed using electrons emitted from an emitter in an electron gun which acts as a fine probe as it interacts with the wafer to inspect the microstructure.

[0007] FIG. 1 is an embodiment of a previous embodiment of an electron gun for generating an electron beam. In the thermal field emission (TFE) mechanism, an extractor controls the emission brightness of the electrons from the tip. A suppressor controls the focus (trajectory) of the electrons. The electrons are accelerated by the anode to a specific beam energy (BE), such as 10 kV, 20 kV, or 30 kV. The design of the anode can vary. For example, the beam limiting aperture (BLA) in FIG. 1 and the holder of the BLA constitute the anode. The anode is mostly grounded. The pole pieces and coils of the magnetic gun lens are usually sealed in air to reduce contamination of the electron emission and the high vacuum that may be required for the electron optics.

[0008] The pole piece of the magnetic gun lens is designed to immerse the electron source in a focusing magnetic field for short focusing distance and small gun lens aberration. Therefore, an equivalent gun lens (GL) is formed between the tip and the BLA, as shown in FIG1 . FIG1 shows a raw beam current (Iraw) in which most of the emitted electrons are blocked by the BLA, and the BLA selects only the central portion of the electrons in a solid angle α to form a wafer inspection application such as inspection and testing of semiconductor wafers. The current through the magnetic lens coil controls the focus of the electron beam selected by the BLA. The BLA selected electron beam can be focused to form a beam crossover (xo) or defocused to illuminate subsequent lenses in the electron beam column optics.

[0009] Figures 2A-2B show previous focused electron beam optics. In order to select a large beam current range (e.g., about 0.1 nA to 500 nA) for various applications using a fixed BLA and column aperture (APT) and to optimize the resolution at the wafer (WF) for each beam current, the four-lens optical configuration in Figures 2A-2B is used. Figure 2A is an optic for use with low beam currents (e.g., about 0.1 nA to 30 nA) and Figure 2B is an optic for use with high beam currents (e.g., about 30 nA to 500 nA).

[0010] In FIG2A , electrons in the raw beam current (Iraw) selected by the BLA are focused by the first condenser lens (CL1) to form a crossover (xo) between CL1 and the APT and then illuminate the APT. The APT selects the beam current (BC) from the raw current Iraw. A second condenser lens (CL2) and an objective lens (OL) are used to optimize the numerical aperture (NA, i.e., the electron beam focus angle at the wafer) to minimize aberration blur (i.e., minimize the combination of lens spherical aberration, lens chromatic aberration, and diffraction aberration, and blur caused by Coulomb interactions between electrons). CL1 and CL2 may be electrostatic lenses or magnetic lenses. The objective lens (OL) may be an immersed magnetic lens combined with an electrostatic retardation field to decelerate electrons from beam energy (BE) to landing energy (LE).

[0011] In FIG2B , the tip-emitted electrons are strongly focused by the gun lens (GL) to form a first crossover (xo1) between BLA and CL1. The BLA selected Iraw in FIG2B is higher than the BLA selected Iraw in FIG2A because the emission angle α in FIG2B is larger than the emission angle α in FIG2A. The APT selects a higher beam current focused at the second crossover (xo2) by CL1 and CL2 selects the best NA to obtain the best resolution in combination with the use of a higher BC.

[0012] Degradation of optical resolution due to Coulomb interactions between electrons may occur in the design of Figures 2A-2B, which may hinder operation.The higher the raw beam current (Iraw), the worse the optical resolution will be.

[0013] Given a BC for a particular application, residual electrons in the beam current of (Iraw-BC) between the BLA and the APT will create additional optical blur at the wafer, which degrades the optical resolution. In one example, given an Iraw of 30nA in FIG. 2A , if the BC is selected to be 1nA for wafer inspection, there are 29nA of residual electrons. These 29nA electrons will create additional Coulomb interaction (CI) blur to degrade the optical resolution. In another example, given an Iraw of 200nA in FIG. 2B , if the BC is selected to be 50nA for wafer inspection of physical deflection, there are 150nA of residual electrons. These 150nA electrons will create additional CI blur to degrade the resolution for use in conjunction with a 50nA BC.

[0014] The Coulomb interaction not only degrades the resolution in the central part of the electron beam spot, but also makes the tail of the beam electron distribution longer and wider, which contaminates the useful signal extracted by the central electrons of the beam.

[0015] Using the design of FIG. 2B , computer simulations using the Monte Carlo method show how the electrons on the wafer are distributed at the best focus of the complete optics from the tip to the wafer, as shown in FIGS. 3A-3B . Given the 100 nA Iraw in FIG. 2B for a 50 nA BC used in conjunction with a 10 kV BE and a 1 keV LE, FIG. 3A shows how the electrons on the wafer are distributed with a long tail and a fuzzy spot. The spot size (or resolution) in the electron optics can be defined using the center portion of the distributed electrons. For example, FWxx is used to define the full width spot size, which includes xx% of the total electrons in the beam current (e.g., FW50→50%). Using FW50, FW75, and FW85 to measure the electron distribution in FIG. 3A , FIG. 3B shows how the center spot size changes with the tail of the electron distribution.

[0016] Therefore, there is a need for improved systems and methods for generating electron beams. Summary of the invention

[0017] In a first embodiment, a system is provided. The system includes: an electron source configured to emit an electron beam; a suppression electrode disposed proximate to the electron source; an extraction electrode disposed proximate to the electron source; a beam-limiting assembly; and at least one pole piece disposed adjacent to the beam-limiting assembly. The beam-limiting assembly defines a first beam-limiting aperture having a first diameter, a second beam-limiting aperture having a second diameter, and a channel between the first beam-limiting aperture and the second beam-limiting aperture. The channel has a third diameter that is greater than the first diameter of the first beam-limiting aperture and the second diameter of the second beam-limiting aperture. The beam-limiting assembly is positioned to receive the electron beam in the first beam-limiting aperture.

[0018] The beam-limiting assembly may include a flange on a surface of the beam-limiting assembly that receives the electron beam from the electron source. The flange is disposed between the electron source and the at least one pole piece.

[0019] The first beam-limiting aperture may be disposed closer to the electron source than the second beam-limiting aperture.

[0020] The beam-limiting aperture may further include a transition region between the first beam-limiting aperture and the channel. The transition region has a diameter that increases from the first diameter to the third diameter. The transition region has a length from 1 mm to 10 mm along the direction of the electron beam. The third diameter may be configured to block most secondary electrons without attenuating primary electrons.

[0021] The first diameter may be 1.5 to 5.0 times the second diameter.

[0022] The system may further include an objective lens, an aperture disposed in a path of the electron beam between the beam limiting assembly and the objective lens, and a condenser lens disposed in the path of the electron beam between the aperture and the beam limiting assembly.

[0023] The system may further include an objective lens, an aperture disposed in a path of the electron beam between the beam limiting assembly and the objective lens, and a condenser lens disposed in the path of the electron beam between the aperture and the objective lens.

[0024] The beam-limiting assembly may define a groove on the surface receiving the electron beam from the electron source. The first beam-limiting aperture is disposed in a base of the groove. The groove has a fourth diameter greater than the first diameter and the second diameter.

[0025] The second beam-limiting aperture may be spaced 5 mm to 10 mm from the first beam-limiting aperture along the path of the electron beam.

[0026] In an example, the third diameter is about 1 mm in diameter and the length of the channel is about 6 mm to 12 mm.

[0027] In a second embodiment, a method is provided. The method includes forming an electron beam and directing the electron beam through a beam limiting assembly. The beam limiting assembly defines a first beam limiting aperture having a first diameter, a second beam limiting aperture having a second diameter, and a passage between the first beam limiting aperture and the second beam limiting aperture. The passage has a third diameter that is greater than the first diameter of the first beam limiting aperture and the second diameter of the second beam limiting aperture. The beam limiting assembly is positioned to receive the electron beam in the first beam limiting aperture.

[0028] In an example, the beam current of the electron beam is from 1 nA to 100 nA.

[0029] The method may further include focusing the electron beam after the electron beam exits the beam limiting assembly to form a crossover.

[0030] The method may further include defocusing the electron beam after the electron beam exits the beam limiting assembly.

[0031] The method may further include directing the electron beam through a condenser lens activated and disposed downstream of the beam limiting assembly along a path of the electron beam, an aperture disposed downstream of the condenser lens along the path of the electron beam, and an objective lens disposed downstream of the aperture along the path of the electron beam. In an example, the beam current of the electron beam may be from 0.1 nA to 20 nA or from 60 nA to 500 nA.

[0032] The method may further include directing the electron beam through an aperture disposed downstream of the beam limiting assembly along a path of the electron beam, a condenser lens activated and disposed downstream of the aperture along the path of the electron beam, and an objective lens disposed downstream of the condenser lens along the path of the electron beam. In an example, the beam current of the electron beam is from 20 nA to 60 nA.

[0033] The method may further include directing the electron beam to a surface of a semiconductor wafer.The beam current selected by the second beam-limiting aperture is equal to the beam current at the surface of the semiconductor wafer.

[0034] The method may further include directing the electron beam through a groove on a surface of the beam-limiting assembly that receives the electron beam from the electron source. The first beam-limiting aperture is disposed in a base of the groove. The groove has a fourth diameter that is larger than the first diameter and the second diameter.

[0035] The electron beam can pass through a channel between the first beam-limiting aperture and the second beam-limiting aperture. The channel has a third diameter that is larger than the first diameter of the first beam-limiting aperture and the second diameter of the second beam-limiting aperture. The third diameter is configured to block most secondary electrons without attenuating primary electrons. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] For a more complete 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, wherein:

[0037] FIG. 1 is an embodiment of a previous embodiment of an electron gun;

[0038] 2A-2B are embodiments of previous embodiments of electron beam optics;

[0039] 3A-3B are computer simulations of electron optical configurations demonstrating the effects of Coulomb interactions between electrons;

[0040] Figure 4 is an embodiment of an electron gun with a dual aperture according to the present disclosure;

[0041] Figure 5 exhibit Figure 4 Electron optics of an electron gun;

[0042] Figures 6A to 6C Instructions for use Figure 4 The optical configuration of the electron gun;

[0043] Figure 7 Demonstrate the use of Figure 4 and Figure 5Improvement of electron gun resolution of the electron gun of the beam spot of the double aperture electron gun;

[0044] Figure 8 Demonstrating the Coulomb interaction between primary electrons (PE) and secondary electrons (SE) in the region between BLA1 and BLA2;

[0045] Fig. 9 Explain the narrow channel between BLA1 and BLA2 for reducing the motion path of secondary electrons;

[0046] Fig.10 is an embodiment of an electron gun having a dual aperture and a recessed anode;

[0047] Fig.11A exhibit Fig.10 Computer simulation of the secondary electron trajectories of the electron gun;

[0048] Fig. 11B exhibit Fig.11A A magnified image of secondary electrons in the;

[0049] Figures 12A to 12F are optical images from experiments using embodiments disclosed herein; and

[0050] Fig.13 is a flow chart of an embodiment of a method according to the present disclosure. DETAILED DESCRIPTION

[0051] Although the claimed subject matter will be described in terms of specific embodiments, other embodiments (including embodiments that do not provide all of the benefits and features set forth herein) are also within the scope of the present disclosure. Various structural, logical, process step, and electronic changes may be made without departing from the scope of the present disclosure. Accordingly, the scope of the present disclosure is defined solely by reference to the appended claims.

[0052] The performance of a focused electron beam device is usually characterized by the spot size (resolution) at a given beam current (throughput). In order to obtain high throughput with high beam current and high resolution with small spot size, one approach is to reduce the Coulomb interaction between electrons in high beam currents after the geometric aberrations of the lens have been minimized.

[0053] Embodiments herein disclose a dual aperture concept for building a high resolution electron gun by reducing the effects of Coulomb interactions as electrons move in the gun region, since distance accumulation blurring is caused by trajectory displacement effects of Coulomb interactions. Simulations and experiments show improvements in electron beam resolution for high beam currents.

[0054] The effect of resolution (spot size) degradation due to Coulomb interaction is called trajectory displacement effect. The electron beam spot blur dTD due to trajectory displacement effect is the distance accumulation effect given by Equation 1 below.

[0055]

[0056] In this equation, z is the optical axis from tip (Z_source) to wafer (Z_wafer), and F is a function of the beam profile trajectory r(z), the beam voltage or beam energy V(z), and the beam current I(z) from tip to wafer.

[0057] For the design of Figures 2A to 2B, the integral in the equation can be divided into two regions. One region is the integral from the tip (Z_source) to the column aperture (Z_APT) and the function F[r(z), V(z), Iraw], and the other region is the integral from the column aperture (Z_APT) to the wafer (Z_wafer) and the function F[r(z), V(z), BC]. If BC is less than Iraw, the shorter the distance (Z_APT-Z_source), the more the CI (Coulomb interaction) blurs d in Equation 1. TD Will be smaller.

[0058] Figure 4 An embodiment of an electron gun with dual apertures that reduces CI blur is illustrated. System 200 includes an electron source 201 that emits an electron beam 204. Electron source 201 may be or include a tip. Suppression electrode 202 and extraction electrode 203 are disposed proximate to electron source 201.

[0059] The beam limiting assembly 205 defines at least two beam limiting apertures. The first beam limiting aperture 206 (also referred to as BLA1) has a first diameter 212. The second beam limiting aperture 207 (also referred to as BLA2) has a second diameter 213. The second diameter 213 can be smaller than the first diameter 212. For example, the first diameter 212 can be 1.5 to 5.0 times the second diameter 213. In an example, the first beam limiting aperture 206 is approximately 2 to 3 times the second beam limiting aperture 207. The first beam limiting aperture 206 is larger than the second beam limiting aperture 207, so the second beam limiting aperture 207 is fully electron illuminated, and the disclosed range provides this effect. The second beam limiting aperture 207 can be positioned close to the electron source 201 to reduce Coulomb interactions. For example, in Figure 4 In the embodiment, the aperture is moved closer to the electron source 201 to reduce the Coulomb interaction effect (i.e., the aperture is moved upward by more than 100 mm or >1 / 4 column length from the original position (OH opening position) to the BLA2 position). In general, the aperture can be moved to a position where the gun magnetic field penetration is weak and the secondary electrons from BLA2 can be mostly cut off by the SE channel wall before they are strongly focused by the penetrating magnetic field of the gun lens. Figure 8 In the example, BLA2 is moved upward and about 5 mm to 10 mm or 5 mm to 20 mm below BLA1.

[0060] In use Figure 4 In the example of FIG. 1 , the first beam limiting aperture 206 has a diameter of 150 μm and the second beam limiting aperture 207 has a diameter of 60 μm. These dimensions can be used to select a beam current from 1 nA to 500 nA. Beam currents from 1 nA to 100 nA are typically used for image formation in inspection and testing. Beam currents from 100 nA to 500 nA are typically used for overflow charging of the wafer for voltage contrast (VC) inspection. The first beam limiting aperture 206 and the second beam limiting aperture 207 can be optimized for a specific application or a specific beam current.

[0061] The beam limiting assembly 205 also includes a channel 215 having a third diameter 214. The third diameter 214 is larger than the first diameter 212 or the second diameter 213. The channel 215 is between the first beam limiting aperture 206 and the second beam limiting aperture 207, thereby connecting the first beam limiting aperture 206 and the second beam limiting aperture 207. The electron beam 204 can pass through the first beam limiting aperture 206, the channel 215 and the second beam limiting aperture 207.

[0062] In an example, the third diameter 214 is about 1 mm in diameter and the length of the channel 15 is about 6 mm to 12 mm (ie, from the first beam-limiting aperture 206 to the second beam-limiting aperture 207). The third diameter 214 can block most SEs without cutting PEs. Figure 4 , Figure 8 and Fig. 9 The length of the channel 15 in the Fig.10 The length of the channel 15 in is about 6 mm.

[0063] like Figure 4 , a first beam limiting aperture 206, a second beam limiting aperture 207, and a channel 215 are in the beam limiting assembly 205. The first beam limiting aperture 206 is disposed closer to the electron source 201 than the second beam limiting aperture 207. Thus, the electron beam 204 enters the first beam limiting aperture 206 before entering the second beam limiting aperture 207.

[0064] The second beam-limiting aperture 207 can provide a function similar to the post aperture (APT) in FIG. 2 . Thus, the function of the post aperture is relocated to the electron gun region, which reduces the distance from the tip (Z_source) to the post aperture (Z_APT). This reduces the effect of Coulomb interactions on resolution. As explained herein, FIG. 3B and Figure 7 The resolution (or spot size) is usually measured using FW50. With the same beam current, FW50 is about 3 divisions in FIG. 3B and Figure 7 is about 1.5 divisions. Thus, using the embodiments disclosed herein, the reduction in spot size is about 2 times, or about 4 times the increase in beam current density. In addition, the tail (FW75 and FW85) is narrowed. By comparing FIG. 3B with Figure 7 , the tail is about 6 divisions versus about 3 divisions for FW75 and about 8 divisions versus about 6.5 divisions for FW85.

[0065] In the theory of Coulomb interactions between electrons, the electron distribution in the beam spot can be divided into Holtsmark distribution and pencil beam distribution. These are all non-Gaussian distributions. The tails of these distributions are longer than the tails of the standard Gaussian distribution. Coulomb interactions produce tails and these Coulomb interactions are worse than Gaussian tails. In a single electron beam system, the electron distribution is usually a pencil beam distribution. In a multi-beam system, the electron distribution is usually a Holtsmark distribution. In the same principle as Equation 1, when the column aperture moves as close to the source as possible in radial direction, the tail of the distribution accumulates over distance, making the tail narrower.

[0066] The beam limiting aperture may include a transition region 210 between the first beam limiting aperture 206 and the channel 215. In the transition region 210, the diameter increases from the first diameter 212 to the third diameter 214. Therefore, the transition region 210 may be angled or curved relative to the inner surface of the first beam limiting aperture 206 or the wall of the channel 215. The transition region 210 may have a length from 1 mm to 10 mm along the direction of the electron beam. The best case is to block most of the secondary electrons approaching the second beam limiting aperture 207, which minimizes the Coulomb interaction between the primary electrons and the secondary electrons, such as Figure 8 To be shown in 11.

[0067] At least one pole piece 208 is disposed adjacent the beam limiting assembly.

[0068] The beam limiting assembly 205 may include a flange 209 on a surface of the beam limiting assembly 205 that receives the electron beam 204 from the electron source 201. Figure 4 , the flange 209 is disposed between the pole piece 208 and the electron source 201. The flange 209 can protect the pole piece 208 from the electron beam 204.

[0069] Figure 4 The electron gun in the system 200 may be used Figure 5 The gun electron optics are described in . Figure 5 The gun lens (GL) in the Figure 4 The combination of the electrostatic acceleration lens and the magnetic focusing lens between the first beam limiting aperture 206 in the BLA2 ( Figure 4The beam current selected by the second beam limiting aperture 207 in the BLA1 is the second raw beam current (Iraw2). The beam current selected by BLA1 is the first raw beam current (Iraw1). The size of BLA1 is larger than the size of BLA2 (e.g., about 1.5 to 5.0 times). The distance between BLA1 and BLA2 can be relatively short, such as a few millimeters. For example, the distance between BLA1 and BLA2 can be from 6 mm to 12 mm. Iraw1 and Iraw2 are controlled by the excitation of the gun magnetic lens. The electron beam after BLA2 can be focused to form a crossover (xo) below BLA2. The electron beam after BLA2 can also be defocused to form a divergent, telecentric, or converging beam to illuminate a subsequent lens.

[0070] Figures 6A to 6C Instructions for use Figure 4 The electron gun and the fixed-size optical configuration of BLA1 and BLA2. Fig. 6A is an exemplary gun optic for low beam current (eg, approximately 0.1 nA to 20 nA). Figure 6B is an exemplary gun optic for medium beam currents (eg, approximately 20 nA to 60 nA). Figure 6C is an exemplary gun optic for high beam current (eg, approximately 60 nA to 500 nA).

[0071] exist Figures 6A to 6C 2 is now an open hole (OH) used only for electron beam blanking. The open hole aperture (OH APT) is designed to be large enough not to cut off any electrons for the full beam current.

[0072] Figures 6A to 6C The electron optical column in still comprises four lenses (or in an embodiment, consists of four lenses). This comprises a gun lens (GL), a first and a second focusing lens (CL1 and CL2) and an objective lens (OL). The open aperture may be disposed in the path of the electron beam between the beam limiting assembly and the objective lens. The focusing lens may be disposed in the path of the electron beam between the aperture and the beam limiting assembly or the aperture and the objective lens.

[0073] Only three lenses may be used for a particular beam current during operation. Fig. 6A and Figure 6B In another example, CL2 is a shutdown operation. Figure 6B In the example, CL1 is the shutdown operation.

[0074] Due to the short distance between BLA1 and BLA2 (e.g., a few millimeters), the distance accumulation blurring due to Coulomb interactions between residual electrons (Iraw1-Iraw2) can be sufficiently reduced that it can be negligible. Therefore, if the open aperture does not cut electrons from the beam current, the final beam current (BC) can be directly equal to or nearly equal to Iraw2.

[0075] therefore, Figures 6A to 6C The electron optical configuration in can remove most or all of the residual electrons below BLA2. Thus, most of the additional blurring in Figures 2 and 3 due to CI track shift effects is removed. Figures 6A to 6C The optical configuration in the image sensor improves the optical resolution and the tail of the electron distribution. Figure 7 Demonstrate improvements in computer simulation using Monte Carlo methods. Figure 7 Demonstrate the use of Figure 4 and Figure 5 The resolution of the electron gun is improved by improving the electron distribution of the beam spot of the double-aperture electron gun.

[0076] For reference, FIG. 3B shows the electron distribution of the beam spot at the wafer using the previous design. Figure 7 Display Figure 6C The electron distribution of the beam spot at the wafer of the optical configuration of FIG3B and FIG3C is used to simulate Figure 7 The conditions are the same. Both conditions are 50nA BC in 100nA Iraw electrons moving with 10kV BE and landing with 1keV LE. Figure 7 The most useful central spot size (FW50) in is only half of the FW50 in FIG. 3B . Figure 7 The tail of the electron distribution in FIG. 3A is much narrower than the tail of the electron distribution in FIG. 3B .

[0077] A large number of chip devices (such as 3D NAND flash, 3D DRAM and 3D logic) can be configured with many memory holes, channel holes, steps and deep trenches. In order to be able to collect the bottom signal of these holes, a narrow primary electron beam with a short tail can help reduce signal contamination from the edge of the hole channel wall and channel surface. Therefore, compared with the resolution and secondary electron / backscattered electron signal quality of Figure 3B, Figure 7 The electron beam provides better resolution and better secondary electron / backscattered electron signal quality.

[0078] Figure 8 This illustrates how Coulomb interactions between primary and secondary electrons can occur in the region between BLA1 and BLA2. Figure 8The trajectories of primary and secondary electrons are shown in Figure 1. The anode current (Ianode) is formed by the emitted electrons from the tip to the plane of BLA1, as selected by the extractor. Figure 8 Only a portion of the Ianode electron trajectory is shown in FIG. The central portion of the anode current electrons passing through BLA1 forms a first raw beam current Iraw1. A portion of the Iraw1 electrons bombards BLA2 with accelerated energy (i.e., beam energy BE). A small portion of most of the central electrons passes through BLA2 to form a second raw beam current Iraw2. Iraw2 may be approximately equal to the final beam current (BC) to the wafer.

[0079] When the Iraw1 electrons bombard BLA2, secondary electrons (SE) are generated in the opposite direction of the primary electrons. These secondary electrons will spirally move to BLA1 at a relatively low speed to form a secondary electron cloud between BLA1 and BLA2. These spiral SE trajectories are formed due to the penetration of the magnetic lens field in the area between BLA1 and BLA2. The lower the position close to BLA2, the weaker the penetrated magnetic field may be. Therefore, the higher the position close to BLA1, the stronger the penetrated magnetic field may be. Therefore, the closer to BLA1, the more spiral motions of secondary electrons with shorter spatial periods, which may be because the penetrated magnetic lens field is stronger. Therefore, the resolution of the primary electron beam may be degraded because these electron trajectories may be displaced by secondary electrons through Coulomb interactions.

[0080] To reduce the secondary electron cloud, the diameter of the channel between BLA1 and BLA2 can be adjusted (e.g., Figure 8 The size d in the channel is used to minimize the Coulomb interaction between PE and SE. When the channel diameter d decreases, the SE cloud tends to decrease. Fig. 9 Displayed in.

[0081] exist Fig. 9 In the embodiment, the channel diameter d is designed to be relatively small (e.g., about 1 mm) so that the channel with a large polar angle (e.g., Fig. 9 Most of the SEs with polar angles of 10 to 90 degrees (in the 1 mm diameter) hit the channel wall before they are focused by the penetrating magnetic field. The 1 mm diameter is chosen so that the channel cuts off all secondary electrons with polar angles greater than 5 degrees, so that the effect of the Coulomb interaction between primary and secondary electrons on the resolution is negligible. Smaller d can block more secondary electrons, but can be more difficult to manufacture with high accuracy and high vacuum pumping.

[0082] The penetrated magnetic field can be quite weak in the bottom region of the channel. Only SEs with small polar angles (e.g., 5 degrees or less) can be focused by the penetrated magnetic field and move spirally upward to BLA1. Therefore, the PE trajectory displacement due to Coulomb interaction can be reduced because the number of SEs moving close to PE in the opposite direction is greatly reduced.

[0083] An embodiment of an electron gun with dual aperture moves the column aperture upward to the gun region and uses post-crossover motion under BLA2 to select the beam current. Most or all of the residual electrons under BLA2 are removed, which can reduce the effect of Coulomb interactions on optical resolution. Previously, the beam current was selected using a pre-crossover method like FIG. 2 , where a crossover or several crossovers are moved in front of the column aperture. A large number of residual electrons remain between the gun BLA and the column aperture. In order to select a large beam current range, the distance between the gun BLA and the column aperture can be long, such as from 100 mm to 200 mm (e.g., 140 mm to 150 mm). As a combined result, the residual electrons distributed in a long distance can induce severe Coulomb interactions and degrade the optical resolution through trajectory displacement effects.

[0084] Embodiments disclosed herein can simplify electron optics in an electron optical column. Previous designs using the front cross BC selection method used a four-lens column as in FIG2. Using the back cross BC selection method, three lenses in the column optics can be used for any given beam current, which simplifies optical alignment and provides improved reliability.

[0085] The embodiments disclosed herein can also minimize the Coulomb interaction between PE and SE. The narrow channel between BLA1 and BLA2 can collect secondary electrons with larger polar angles (e.g., 10 to 90 degrees) before they are focused to travel long distances. A concave anode for reducing the distance between BLA1 and BLA2 can be used so that even secondary electrons with small polar angles (e.g., 5 degrees or less) are highly compressed to travel a short action path.

[0086] The gun magnetic lens field can cause the SE to spiral at a low speed, thus Figure 8 SE clouds are formed in the region between BLA1 and BLA2 in FIG. To avoid strong penetration of the magnetic lens field near BLA1, a concave anode can be used, such as Fig.10 . The beam limiting assembly defines a groove 211 on a surface that receives the electron beam 204 from the electron source. A first beam limiting aperture 206 (BLA1) is disposed in the base of the groove 211. The groove has a fourth diameter that is greater than the first and second diameters of the BLA. The fourth diameter may also be greater than the third diameter of the channel.

[0087] The groove 211 allows the acceleration field from the extractor to the anode to fully penetrate the surface of BLA1, so that the low energy SE from BLA1 moves in a spiral motion of about 1 mm ( Figures 11A to 11B) are immediately repelled back to BLA1 to reduce the Coulomb interaction between PE and SE. The groove 211 can also compress the length of the channel between BLA1 and BLA2 and restrict the SE from BLA2 to move only near BLA1 where the gun lens magnetic field penetration is relatively weak.

[0088] Fig.10 The BLA1 in the PDMS is shifted downward to compress the distance between BLA1 and BLA2. This can also compress the spiral motion distance of the SE generated at BLA2 due to the electron bombardment of Iraw1.

[0089] Fig.10 The diameter (d) of the channel between BLA1 and BLA2 in the embodiment of the present invention may be equal to or even smaller than Figure 4 The diameter (d) in the concave anode is reduced because the channel is shorter. Computer simulation can be used to design the inner diameter and conical cut of the concave anode to fully allow the accelerating field (from the extractor to the anode) to penetrate to the top surface of the BLA1 and reject secondary electrons generated by the bombardment of the anode current electrons on the BLA1.

[0090] Fig.11A Display usage Fig.10 The reduction of secondary electrons in an electron gun with a concave anode and a dual aperture. Using computer simulation, Fig.11A Draw the secondary electron trajectories in . Fig. 11B Show Fig.11A A zoomed-in view of the SE trajectory in .

[0091] The spirally moving SEs emitted from BLA1 due to the anode current electron bombardment are immediately repelled back to the BLA1 surface by the acceleration field. The height of the BLA1-SE cloud can be only less than 1 mm. According to the distance accumulation principle in Equation 1, this height can negligibly affect the PE trajectory displacement through Coulomb interaction.

[0092] Most of the spirally moving SEs emitted from BLA2 due to electron bombardment by the Iraw1 current diverge and hit the narrow channel wall within about 1 mm height. Fig. 11B As shown in FIG. 1 , SEs with polar angles within about 10 to 90 degrees are immediately collected by the channel wall, and only a small portion of SEs with polar angles equal to or less than 5 degrees spiral upward to BLA2. Since the distance between BLA1 and BLA2 is reduced using a concave anode, when comparing Fig. 11B The SE trajectory in Fig. 9 When the SE trajectory in the middle is compressed, the spiral moving SE is compressed with fewer spiral periods. This can prevent the secondary electrons from escaping BLA2, such as Fig. 9 or Fig. 11B Displayed in.

[0093] Figures 12A to 12FExperimental results are presented using a range of beam currents (approximately 1 nA to 100 nA) to evaluate defect review, physical defects, hot spot detection, and voltage contrast inspection. Figures 12A to 12F The images are for 1nA, 3nA, 10nA, 50nA, 75nA and 100nA beam currents. For these low (landing) energy (1keV) applications, Figures 12A to 12F The good image resolution at high BC (eg, about 50 nA to 100 nA) confirms that the Coulomb interactions between electrons have been greatly reduced using the embodiments disclosed herein.

[0094] The experimental results are beneficial for high beam currents (e.g., from 50nA to 100nA) used for voltage contrast wafer inspection. Figure 7 ), so the experimental image or resolution degrades quite slowly as the beam current increases up to 100 nA. Conventional CDSEM equipment or inspection equipment can provide high resolution because the beam current is low. In an example, the beam current in a CDSEM is about 0.1 nA or less, and the beam current in an inspection tool is about 0.1 nA to 5 nA. The embodiments disclosed herein can provide high resolution for wafer inspection (specifically, voltage contrast detection) using high beam currents up to 100 nA.

[0095] Fig.13 300. The present invention is a flow chart of an embodiment of a method 300 that may use any of the embodiments disclosed herein. At 301, an electron beam is formed. At 302, the electron beam is directed through a beam limiting assembly. The beam limiting assembly defines a first beam limiting aperture having a first diameter, a second beam limiting aperture having a second diameter, and a channel between the first beam limiting aperture and the second beam limiting aperture. The channel has a third diameter that is greater than the first diameter of the first beam limiting aperture and the second diameter of the second beam limiting aperture. The beam limiting assembly is positioned to receive the electron beam in the first beam limiting aperture.

[0096] The electron beam can be focused after the electron beam exits the beam limiting assembly to form a crossover. The electron beam can also be defocused after the electron beam exits the beam limiting assembly.

[0097] In an example, the electron beam may also be directed through a focusing lens activated and disposed downstream of the beam limiting assembly along the path of the electron beam, an aperture disposed downstream of the focusing lens along the path of the electron beam, and an objective lens disposed downstream of the aperture along the path of the electron beam. The beam current of the electron beam may be from 0.1 nA to 20 nA or from 60 nA to 500 nA.

[0098] In another example, the electron beam may be directed through an aperture disposed downstream of the beam limiting assembly along the path of the electron beam, a condenser lens activated and disposed downstream of the aperture along the path of the electron beam, and an objective lens disposed downstream of the condenser lens along the path of the electron beam. The beam current of the electron beam may be from 20 nA to 60 nA.

[0099] The electron beam may be directed to a surface of the semiconductor wafer. The beam current selected by the second beam-limiting aperture may be equal to the beam current at the surface of the semiconductor wafer.

[0100] The electron beam can be guided through a groove on a surface of the beam limiting assembly that receives the electron beam from the electron source. The first beam limiting aperture is disposed in a base of the groove. The groove has a fourth diameter greater than the first diameter and the second diameter.

[0101] Although described with respect to electron beams, embodiments disclosed herein may also be used with ion beams or particle beams.

[0102] Although the present disclosure has been described with respect to one or more specific embodiments, it should be understood that other embodiments of the present disclosure may be made without departing from the scope of the present disclosure. Therefore, the present disclosure is deemed to be limited only by the appended claims and reasonable interpretations thereof.

Claims

1. A system comprising: an electron source configured to emit an electron beam; a suppression electrode disposed proximate to the electron source; an extraction electrode disposed proximate to the electron source; a beam limiting assembly defining a first beam limiting aperture having a first diameter, a second beam limiting aperture having a second diameter, and a passage between the first beam limiting aperture and the second beam limiting aperture, wherein the passage has a third diameter greater than the first diameter of the first beam limiting aperture and the second diameter of the second beam limiting aperture, wherein the beam limiting assembly is positioned to receive the electron beam in the first beam limiting aperture, wherein the first diameter is 1.5 to 5.0 times the second diameter, wherein the beam limiting assembly further includes a transition region between the first beam limiting aperture and the passage, wherein the transition region has a diameter that increases from the first diameter to the third diameter; and At least one pole piece is disposed adjacent the beam limiting assembly.

2. The system of claim 1, wherein the beam-limiting assembly includes a flange on a surface of the beam-limiting assembly that receives the electron beam from the electron source, wherein the flange is disposed between the electron source and the at least one pole piece.

3. The system of claim 1, wherein the first beam-limiting aperture is disposed closer to the electron source than the second beam-limiting aperture. 4 . The system of claim 1 , wherein the transition region has a length along the direction of the electron beam from 1 mm to 10 mm.

5. The system of claim 4, wherein the third diameter is configured to block most secondary electrons without attenuating primary electrons.

6. The system of claim 1, further comprising an objective lens, an aperture disposed in a path of the electron beam between the beam limiting assembly and the objective lens, and a focusing lens disposed in the path of the electron beam between the aperture and the beam limiting assembly.

7. The system of claim 1, further comprising an objective lens, an aperture disposed in a path of the electron beam between the beam limiting assembly and the objective lens, and a condenser lens disposed in the path of the electron beam between the aperture and the objective lens.

8. The system of claim 1, wherein the beam-limiting assembly defines a groove on a surface that receives the electron beam from the electron source, wherein the first beam-limiting aperture is disposed in a base of the groove, and wherein the groove has a fourth diameter greater than the first diameter and the second diameter.

9. The system of claim 1, wherein the second beam-limiting aperture is spaced 5 mm to 10 mm from the first beam-limiting aperture along the path of the electron beam.

10. The system of claim 1, wherein the third diameter is about 1 mm in diameter and the length of the channel is from 6 mm to 12 mm.

11. A method comprising: forming an electron beam; and Directing the electron beam through a beam limiting assembly, wherein the beam limiting assembly defines a first beam limiting aperture having a first diameter, a second beam limiting aperture having a second diameter, and a channel between the first beam limiting aperture and the second beam limiting aperture, wherein the channel has a third diameter that is greater than the first diameter of the first beam limiting aperture and the second diameter of the second beam limiting aperture, wherein the beam limiting assembly is positioned to receive the electron beam in the first beam limiting aperture, and wherein the first diameter is 1.5 to 5.0 times the second diameter, wherein the beam limiting assembly further includes a transition region between the first beam limiting aperture and the channel, wherein the transition region has a diameter that increases from the first diameter to the third diameter.

12. The method of claim 11, further comprising focusing the electron beam after the electron beam exits the beam limiting assembly to form a crossover.

13. The method of claim 11, further comprising defocusing the electron beam after the electron beam exits the beam limiting assembly. The method according to claim 11 , wherein a beam current of the electron beam is from 1 nA to 100 nA.

15. The method of claim 11, further comprising directing the electron beam through a focusing lens activated and disposed downstream of the beam limiting assembly along a path of the electron beam, an aperture disposed downstream of the focusing lens along the path of the electron beam, and an objective lens disposed downstream of the aperture along the path of the electron beam. 16 . The method according to claim 15 , wherein a beam current of the electron beam is from 0.1 nA to 20 nA or from 60 nA to 500 nA.

17. The method of claim 11, further comprising directing the electron beam through an aperture disposed downstream of the beam limiting assembly along a path of the electron beam, a focusing lens activated and disposed downstream of the aperture along the path of the electron beam, and an objective lens disposed downstream of the focusing lens along the path of the electron beam. The method according to claim 17 , wherein the beam current of the electron beam is from 20 nA to 60 nA.

19. The method of claim 11, further comprising directing the electron beam to a surface of a semiconductor wafer, wherein a beam current selected by the second beam-limiting aperture is equal to a beam current at the surface of the semiconductor wafer.

20. The method of claim 11, further comprising directing the electron beam through a groove on a surface of the beam-limiting assembly that receives the electron beam from an electron source, wherein the first beam-limiting aperture is disposed in a base of the groove, and wherein the groove has a fourth diameter greater than the first diameter and the second diameter.

21. The method of claim 11, wherein the electron beam passes through the channel, and wherein the third diameter is configured to block most secondary electrons without attenuating primary electrons.

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