Electron beam inspection device

Abstract

The present invention provides an electron beam inspection device that can reduce the potential change of the positive electrode immediately after blanking is released. [Solution] The electron beam inspection apparatus includes an objective lens that images a primary electron beam onto a sample. A negative potential is applied to the sample. The objective lens has an electromagnetic lens that forms a magnetic field in the gap between the inner diameter end of the Wehnert electrode and the inner diameter end of the yoke, and has a Wehnert electrode positioned opposite the sample as viewed from the Wehnert electrode, to which a negative potential is applied, and a positive potential electrode positioned opposite the sample as viewed from the Wehnert electrode, to which a positive potential is applied, and a ground electrode positioned between the Wehnert electrode and the positive potential electrode to prevent backscattered electrons emitted from the sample from colliding with the positive potential electrode, and is electrically grounded.

Description

Technical Field

[0001] The present invention relates to an electron beam inspection apparatus.

Background Art

[0002] An electron beam inspection apparatus that irradiates a sample with an electron beam and observes the electron beam emitted from the sample is known (for example, Patent Document 1). The electron beam inspection apparatus includes a primary optical system that irradiates a sample with a primary electron beam and a secondary optical system that detects a secondary electron beam generated from the sample as an image.

[0003] By configuring the primary electron beam in the primary optical system with a plurality of electron beams, it is possible to achieve high throughput. In such an electron beam inspection apparatus, the electron beam emitted from the electron source passes through a multi-beam generation mechanism provided with a plurality of apertures, thereby generating a primary electron beam composed of a plurality of primary electrons. The generated primary electron beam is individually focused by a transfer lens and an objective lens, and is discretely irradiated at equal intervals to a plurality of locations on the sample. Further, the primary electron beam is deflected so as to two-dimensionally scan the sample by a scan deflector disposed between the transfer lens and the objective lens. As a result, the discretely irradiated primary electron beam is evenly irradiated onto the sample.

[0004] As a means for switching between a state of irradiating a sample with a plurality of electron beams and a state of not irradiating, a blanking is known in which a predetermined voltage or current is applied to a deflector element disposed upstream of the objective lens to deflect a plurality of electron beams, and the plurality of electron beams are entirely irradiated onto a Faraday cup disposed between the deflector and the objective lens, thereby preventing the plurality of electron beams from irradiating the sample. As the deflector used here, a dedicated deflector for blanking may be disposed, or it may be used in combination with a deflector for axis adjustment or scanning.

[0005] De-blanking means reducing the predetermined voltage or current applied to the deflector for blanking, thereby allowing the multiple electron beams that were irradiating the Faraday cup to irradiate the sample.

[0006] When multiple electron beams are irradiated onto a sample, electrons with a wide range of energies, from near zero to the irradiation energy, are emitted from the sample. Generally, among the electrons emitted from the sample, those with energies from 50 eV to the irradiation energy are called backscattered electrons, and those with energies from near zero to 50 eV are called secondary electrons.

[0007] Furthermore, Patent Document 1 discloses that a ground electrode is placed upstream of the surface electric field control electrode to prevent the electric field, which consists of a negative potential applied to the surface electric field control electrode or the sample, from leaking upstream. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Patent No. 5886663 [Overview of the project] [Problems that the invention aims to solve]

[0009] Incidentally, an electromagnetic field superposition type objective lens is known that uses a positive potential electrode along with a magnetic field lens. The positive potential electrode increases the potential near the point where multiple electron beams converge, thereby increasing the electron energy and reducing the electron residence time. This reduces the space charge effect and suppresses the increase in beam spot size, especially when the beam current is large. The relative permeability of the material of the positive potential electrode is about the same as that of vacuum, and it does not exert any magnetic lensing effect on the electron beam passing through the inner diameter. On the other hand, the presence of the positive potential electrode does produce some degree of electrostatic lensing effect on the electron beam passing through the inner diameter.

[0010] In conventional objective lens configurations, secondary electrons emitted from the sample are focused by the magnetic field of the objective lens and therefore do not collide with the positive potential electrode. On the other hand, backscattered electrons have high energy and are not sufficiently focused by the magnetic field of the objective lens, so many of them collide with the positive potential electrode.

[0011] Therefore, immediately after the blanking is released, the irradiation of the positive potential electrode with backscattered electrons instantaneously transitions from zero to a state where a certain number of electrons are irradiated. Since the positive potential of the positive potential electrode is due to the accumulation of positive charge on the electrode, an instantaneous change in the incoming electrons causes an instantaneous change in the potential of the positive potential electrode.

[0012] Changes in the potential of the positive electrode alter the electrostatic lens action, causing a focus shift in the objective lens, resulting in an increase in beam spot size and a decrease in secondary electron detection efficiency. Furthermore, inspection data obtained while the potential returns to its original value cannot be used, leading to a decrease in inspection throughput.

[0013] The present invention has been made in consideration of the above points. The object of the present invention is to provide an electron beam inspection device that can reduce the potential change of the positive potential electrode immediately after blanking is released. [Means for solving the problem]

[0014] An electron beam inspection apparatus according to a first aspect of the present invention is: An electron beam inspection apparatus comprising: a primary optical system that irradiates a sample on a stage with a primary electron beam composed of multiple primary electrons; and a secondary optical system that detects a secondary electron beam composed of multiple secondary electrons emitted from the sample irradiated with the primary electron beam using a detector, The system has an objective lens that images the primary electron beam onto the sample, A negative potential is applied to the sample. The aforementioned objective lens is, The device comprises a Wehnert electrode positioned opposite the sample to which a negative potential is applied, and a yoke positioned on the opposite side from the sample as seen from the Wehnert electrode, magnetically connected to the Wehnert electrode and electrically insulated, wherein an electromagnetic lens forms a magnetic field in the gap between the inner diameter end of the Wehnert electrode and the inner diameter end of the yoke. A positive potential electrode is positioned on the opposite side of the sample from the Wehnelt electrode, to which a positive potential is applied, A ground electrode is placed between the Wehnelt electrode and the positive potential electrode and is electrically grounded to prevent backscattered electrons emitted from the sample from colliding with the positive potential electrode, It has.

[0015] In this configuration, since a ground electrode is placed between the Wehnelt electrode and the positive potential electrode, a large number of backscattered electrons irradiate the ground electrode, reducing the irradiation of the positive potential electrode with backscattered electrons. This reduces the potential change of the positive potential electrode immediately after blanking is released, thereby reducing the increase in beam spot size and the decrease in secondary electron detection efficiency caused by the potential change of the positive potential electrode.

[0016] An electron beam inspection apparatus according to a second aspect of the present invention is an electron beam inspection apparatus according to a first aspect, The inner diameter of the ground electrode is smaller than the inner diameter of the positive potential electrode.

[0017] An electron beam inspection apparatus according to a third aspect of the present invention is an electron beam inspection apparatus according to a second aspect, When the inner diameter of the positive potential electrode is set to 1, the inner diameter of the ground electrode is 0.7 or less.

[0018] An electron beam inspection apparatus according to a fourth aspect of the present invention is an electron beam inspection apparatus according to a second or third aspect, When the inner diameter of the positive potential electrode is set to 1, the inner diameter of the ground electrode is 0.2 or greater. [Effects of the Invention]

[0019] According to the present invention, it is possible to reduce the potential change of the positive electrode immediately after the blanking is released.

Brief Description of the Drawings

[0020] [Figure 1] FIG. 1 is a diagram showing a schematic configuration of an electron beam inspection apparatus according to an embodiment. [Figure 2] FIG. 2 is a diagram showing an enlarged configuration of an objective lens according to an embodiment. [Figure 3] FIG. 3 is a diagram showing an enlarged configuration of a conventional objective lens. [Figure 4] FIG. 4 is a graph showing an example of the frequency distribution of electron collision positions.

Embodiments for Carrying Out the Invention

[0021] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following description and the drawings used in the following description, the same reference numerals are used for parts that can have the same configuration, and duplicate descriptions are omitted.

[0022] FIG. 1 is a diagram showing a schematic configuration of an electron beam inspection apparatus 1 according to an embodiment. Note that the sample 30 to be inspected by the electron beam inspection apparatus 1 may be, for example, a silicon wafer, a glass mask, a semiconductor substrate, a semiconductor pattern substrate, or a substrate having a metal film.

[0023] As shown in FIG. 1, the electron beam inspection apparatus 1 includes a primary optical system 10 (also called an irradiation system or a multi-beam optical system) that irradiates an electron beam onto a sample 30 on a stage (not shown), and a secondary optical system 20 (also called an imaging system or a projection optical system) that forms an enlarged image of signal electrons (secondary electrons, reflected electrons, etc.) from the sample on a detector 28.

[0024] The primary optical system 10 focuses and irradiates multiple locations on the sample 30 with a primary electron beam composed of multiple primary electrons, and includes an electron source 11, a multi-beam generation mechanism 12, a transfer lens 13, a deflector 14, a Faraday cup 15, a beam separator 16, and an objective lens 15.

[0025] The electron source 11 is provided at one end of a column (vacuum tube) (not shown) and emits an electron beam into the column. As the electron source 11, for example, a photoelectron source having a laser light source and a photocathode, as described in Japanese Patent Application Publication No. 2012-253007, can be used. The photocathode structure used in the photoelectron source can achieve high efficiency. Note that the electron source 11 is not limited to a photoelectron source as long as it can emit an electron beam, for example, an electron gun such as LaB6 can also be used.

[0026] The electron beam emitted from the electron source 11 is accelerated by an accelerator (not shown) as appropriate and broadened by a lens (not shown) before being incident on the multi-beam generation mechanism 12.

[0027] The multi-beam generation mechanism 12 has multiple apertures formed to allow the multi-beam to pass through. The electron beam from the electron source 11 passes through the multiple apertures formed in the multi-beam generation mechanism 12, generating a primary electron beam composed of multiple primary electrons. The generated primary electron beam is individually focused by the transfer lens 13 and the objective lens 17, and irradiates multiple locations on the sample 30 placed on a stage (not shown) at equal intervals.

[0028] In the example shown in Figure 1, a primary electron beam composed of multiple primary electrons is generated using a multi-beam generation mechanism 12 having multiple apertures. However, the configuration for generating the primary electron beam is not limited to using a multi-beam generation mechanism 12 having multiple apertures. For example, instead of an electron source 11 and a multi-beam generation mechanism 12, multiple electron sources may be provided, and a primary electron beam composed of multiple primary electrons may be generated by each electron source emitting an electron beam. As for the multi-beam generation mechanism 12, in addition to those having multiple apertures, field emitter arrays (see, for example, Figure 10 of JP 2005-213567) and multi-tube configurations that bundle multiple single primary electron beam tubes (see, for example, Figures 3, 8, and 9 of JP 2005-197121) are known, but are not limited to these.

[0029] Although Figure 1 schematically shows three primary electron beams, there are no particular restrictions on the number of beams; for example, there could be a few, 1000, or even more.

[0030] The deflector 14 is positioned between the transfer lens 13 and the objective lens 17, and deflects the direction of the primary electron beam in the XY direction. As a result, the discretely irradiated primary electron beam is scanned evenly in two dimensions over the sample 30.

[0031] The Faraday cup 15 is positioned between the deflector 14 and the objective lens 17. By applying a predetermined voltage or current to the deflector 14, which is positioned upstream of the objective lens 17, multiple electron beams are deflected and the entire beam is directed onto the Faraday cup 15, thereby preventing the sample 30 from being irradiated by multiple electron beams (blanking). Conversely, by reducing the predetermined voltage or current applied to the deflector 14 for blanking, the multiple electron beams that were irradiating the Faraday cup 15 are directed onto the sample 30 (blanking release). In Figure 1, the deflector 14 is shown to have both scanning and blanking functions simultaneously, but a separate deflector dedicated solely to blanking and its release may be positioned in addition to the deflector 14.

[0032] The secondary optical system 20 detects a secondary electron beam, which is composed of multiple secondary electrons emitted from a sample 30 irradiated with a primary electron beam, using a detector 28. The secondary optical system 20 includes an objective lens 17, a beam separator 16, a beam bender 21, a first relay lens 22, a second relay lens 23, a dispersion corrector 24, a field lens 25, an aperture diaphragm 26, a projection lens 27, and a detector 28. The objective lens 17 and the beam separator 16 are shared with the primary optical system 10.

[0033] The secondary electron beam from the sample 30 is focused by the objective lens 17. Then, the secondary electron beam is bent in a direction different from that of the primary optical system 10 by the beam separator 15, which forms a superposition field of electric and magnetic fields, and then bent further by the beam bender 21.

[0034] The first relay lens 22 and the second relay lens 23 are adjusted so that the secondary electron beam forms an image of a constant size at a constant position near the main surface of the field lens 25, regardless of the potential of the sample 30. By providing these first relay lenses 22 and the second relay lens 23, a wide range of sample potentials can be accommodated. Furthermore, the secondary electron beam that has passed through the beam separator 16 and beam bender 21 will experience dispersion due to its wide energy range. Although not shown in Figure 1, a dispersion corrector may be placed between the beam bender 21 and the field lens 25 to compensate for the dispersion.

[0035] The field lens 25 generates an electric or magnetic field near the position of the aperture diaphragm 26 to adjust the trajectory of the secondary electron beam so that the multiple secondary electrons constituting the secondary electron beam are closest to each other at the optical axis center. In other words, the aperture diaphragm 26 is positioned so that the multiple secondary electrons are closest to each other at the optical axis center.

[0036] The aperture diaphragm 26 has an opening, and only the secondary electron beam that passes through the opening reaches the projection lens 27. This defines the opening angle of the secondary electron beam.

[0037] The projection lens 27 images the secondary electron beam that has passed through the aperture diaphragm 26 onto the detector 28. The detector 28 has multiple detectors corresponding to multiple secondary electron beams. This consists, for example, of a scintillator and multiple photodetectors corresponding to multiple secondary electron beams, which are placed in the subsequent stage. By determining the pixel gradation corresponding to the individual current values ​​of the multiple detectors, multiple images can be acquired simultaneously, and the sample surface can be inspected using these multiple images.

[0038] Figure 2 is an enlarged view showing the configuration of an objective lens 17 according to one embodiment. As shown in Figure 2, the objective lens 17 according to this embodiment is an electromagnetic field superimposed type objective lens and has an electromagnetic lens 40 and a positive potential electrode 44.

[0039] The electromagnetic lens 40 has an annular shape and is positioned opposite the sample 30. It includes a Wennelt electrode 41 made of a high-permeability material, and a yoke 42 positioned on the opposite side of the Wennelt electrode 41 from the sample 30 (i.e., the Wennelt electrode 41 is sandwiched between the sample 30 and the yoke 42). The yoke 42 is magnetically connected to the Wennelt electrode 41 and electrically insulated from the Wennelt electrode 41. A negative potential is applied to the sample 30, and a potential equal to or several kV lower than the potential applied to the sample 30 (both negative potentials) is applied to the Wennelt electrode 41. The yoke 42 is electrically grounded.

[0040] A coil 43 is positioned inside the yoke 42. The coil 43 is wound circumferentially with respect to the optical axis. When an electric current is passed through the coil 43 to generate a magnetic field, a magnetic flux is created that connects from the yoke 42 to the Wehnelt electrode 41, and a magnetic field (lens magnetic field) is formed in the gap between the inner diameter end 41a of the Wehnelt electrode 41 and the inner diameter end 42a of the yoke 42.

[0041] The positive potential electrode 44 has a cylindrical or annular shape and is positioned on the inner diameter side of the yoke 42. The positive potential electrode 44 is made of a non-magnetic metal (for example, copper). The positive potential electrode 44 increases the electron energy and reduces the electron residence time by making the space potential near the point where multiple electron beams converge positive. This reduces the space charge effect and suppresses the increase in beam spot size, especially when the beam current is large.

[0042] As shown in Figure 2, the objective lens 17 according to this embodiment further includes a ground electrode 45. The ground electrode 45 has an annular shape and is electrically grounded. The ground electrode 45 is positioned between the Wehnelt electrode 41 and the positive potential electrode 44 to prevent backscattered electrons 50 emitted from the sample 30 from colliding with the positive potential electrode 44. The ground electrode 45 is made of a non-magnetic metal (for example, copper).

[0043] The grounding electrode 45 is positioned along the equipotential line (plane) of the grounding potential of the spatial potential formed between the positive potential electrode 44 to which a positive potential is applied and the Wehnelt electrode 41 to which a negative potential is applied.

[0044] Incidentally, as mentioned in the section on the problems the invention aims to solve, in the configuration of a conventional objective lens 144 (see Figure 3) that does not have a ground electrode 45, secondary electrons emitted from the sample 30 are focused by the magnetic field of the objective lens 17 and therefore do not collide with the positive potential electrode 44. On the other hand, because the backscattered electrons 50 have high energy, they are not sufficiently focused by the magnetic field of the objective lens 17 and many of them collide with the positive potential electrode 44.

[0045] Therefore, immediately after the blanking is released, the irradiation of the positive potential electrode 44 with backscattered electrons 50 instantaneously transitions from zero to a state where a certain number of electrons are irradiated. Since the positive potential of the positive potential electrode 44 is due to the accumulation of positive charge on the electrode, an instantaneous change in the incoming electrons causes an instantaneous change in the potential of the positive potential electrode 44.

[0046] Therefore, in the conventional objective lens 144 configuration, changes in the potential of the positive electrode 44 cause a focus shift in the objective lens 144, resulting in an increase in beam spot size and a decrease in secondary electron detection efficiency. Furthermore, inspection data obtained while the potential returns to its original value cannot be used, thus reducing the inspection throughput.

[0047] In contrast, according to this embodiment, as shown in Figure 2, a ground electrode 45 is positioned between the Wehnelt electrode 41 and the positive potential electrode 44. As a result, a large number of backscattered electrons 50 emitted from the sample 30 are irradiated onto the ground electrode 45, reducing the irradiation of the positive potential electrode 44 with backscattered electrons 50. This reduces the potential change of the positive potential electrode 44 immediately after blanking is released, thereby reducing the increase in beam spot size and the decrease in secondary electron detection efficiency due to the potential change of the positive potential electrode 44. Since the backscattered electrons 50 that collide with the ground electrode 45 are quickly discharged, the potential change of the ground electrode 45 is much smaller than the potential change of the positive potential electrode 44, and even if the potential changes, its recovery is much faster than the recovery of the potential of the positive potential electrode 44.

[0048] Backscattered electrons 50 emitted from the sample 30 fly in a manner that spreads out from the optical axis. Since the ground electrode 45 does not have a mechanism to actively collect backscattered electrons 50, many of the backscattered electrons that pass through the hole in the ground electrode 45 may collide with the positive potential electrode 44. In particular, backscattered electrons 50 with low initial energy will have reduced collection efficiency at the ground electrode 45, so it is desirable to use a ground electrode 45 with a smaller inner diameter. For example, the inner diameter of the ground electrode 45 may be smaller than the inner diameter of the positive potential electrode 44.

[0049] By making the inner diameter of the ground electrode 45 sufficiently smaller than the inner diameter of the positive potential electrode 44, the irradiation of backscattered electrons 50 to the positive potential electrode 44 can be reduced more effectively. Therefore, when the inner diameter of the positive potential electrode 44 is set to 1, the inner diameter of the ground electrode 45 is preferably 0.7 or less.

[0050] On the other hand, collisions of secondary electrons with the ground electrode 45 are undesirable because they lead to a decrease in secondary electron detection efficiency. Therefore, when the inner diameter of the positive potential electrode 44 is set to 1, it is preferable that the inner diameter of the ground electrode 45 be 0.2 or greater.

[0051] Next, a specific example of this embodiment will be described.

[0052] When the acceleration energy of the primary electron beam is 30 keV and the potential of the sample 30 is -27 kV, electrons with energies of 0 to 3 keV are emitted from the sample 30. The inventors of this invention performed electron trajectory calculations for the electrons with the maximum energy of 3 keV, assuming that a predetermined number of electrons were emitted from the sample 30 at an opening angle according to Lambert's cosine law, in both the configuration of the embodiment according to this embodiment (see Figure 2) and the configuration of a comparative example without a ground electrode 45 (see Figure 3), and determined the frequency distribution of electron collision positions. Figure 4 is a graph showing an example of the frequency distribution of electron collision positions. Here, the calculations were performed under the condition that +30 kV is applied to the positive potential electrode 44. In the configuration of the embodiment, when the inner diameter of the positive potential electrode 44 is set to 1, the inner diameter of the ground electrode 45 was set to 2 / 3.

[0053] As shown in Figure 4, when the total number of electrons is set to 100, in the comparative example configuration without a ground electrode 45, more than 80 backscattered electrons 45 collide with the positive potential electrode 44, whereas in the configuration of the embodiment according to this example, it was confirmed that almost all of the backscattered electrons 45 can collide with the ground electrode 45.

[0054] Although embodiments of the present invention have been described above by example, the scope of the present invention is not limited to these, and it is possible to modify and transform them according to the purpose within the scope described in the claims. Furthermore, each embodiment can be appropriately combined as long as the processing content is not contradictory. [Explanation of Symbols]

[0055] 1. Electron beam inspection device 10 Primary optical system 11 Electron source 12. Multibeam generation mechanism 13 Transfer Lens 14 Deflector 15 Faraday Cup 16 Beam Separator 17 Objective lens 20 Secondary optical system 21 Beam Vendor 22 First relay lens 23. Second relay lens 24 Dispersion corrector 25 Field Lens 26 Aperture diaphragm 27 Projection lens 28 detectors 30 samples 40 Electromagnetic Lenses 41 Wehnert electrode 42 York 43 coils 44 Positive electrode 45 Ground electrode 50 backscattered electrons

Claims

1. An electron beam inspection apparatus comprising: a primary optical system that irradiates a sample on a stage with a primary electron beam composed of multiple primary electrons; and a secondary optical system that detects a secondary electron beam composed of multiple secondary electrons emitted from the sample irradiated with the primary electron beam using a detector, The system has an objective lens that images the primary electron beam onto the sample, A negative potential is applied to the sample. The aforementioned objective lens is, An electromagnetic lens having a Wehnert electrode positioned opposite the sample, made of a high-permeability material and to which a negative potential is applied, and a yoke positioned on the opposite side from the sample as seen from the Wehnert electrode, magnetically connected to the Wehnert electrode and electrically insulated, and forming a magnetic field in the gap between the inner diameter end of the Wehnert electrode and the inner diameter end of the yoke, A positive potential electrode is positioned on the opposite side of the sample from the Wehnelt electrode, to which a positive potential is applied, A ground electrode is placed between the Wehnelt electrode and the positive potential electrode and is electrically grounded to prevent backscattered electrons emitted from the sample from colliding with the positive potential electrode, Having, Electron beam inspection device.

2. The inner diameter of the ground electrode is smaller than the inner diameter of the positive potential electrode. The electron beam inspection apparatus according to claim 1.

3. When the inner diameter of the positive potential electrode is set to 1, the inner diameter of the ground electrode is 0.7 or less. The electron beam inspection apparatus according to claim 2.

4. When the inner diameter of the positive potential electrode is set to 1, the inner diameter of the ground electrode is 0.2 or greater. The electron beam inspection apparatus according to claim 2 or 3.

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