Generation of Multiple Electron Beams Using a Photoelectric Cathode Film

Abstract

The electron beam device includes a laser and a photocathode film. The photocathode film has a front side and a back side, and emits a plurality of electron beamlets when irradiated from the back side using the laser. The electron beam device further includes an electrode that extracts the plurality of electron beamlets from the front side of the photocathode film and controls the shape of the plurality of electron beamlets.

Description

Technical Field

[0001] The present disclosure relates to electron optics, and more specifically, to generating a plurality of electron beams within an electron beam apparatus.

Background Art

[0002] Scanning electron microscopes (SEMs) have long been used for inspection applications such as semiconductor wafer inspection. Conventionally, SEMs have had a single electron beam. However, more recently, SEMs with a plurality of electron beams (i.e., electron beamlets) have been developed. The plurality of electron beamlets can be generated from a single electron source by splitting a global electron beam into electron beamlets using an aperture array. The aperture array may be accompanied by a micro aberration correction device array, a micro deflection device array, and a micro lens array.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0004] The throughput of a SEM (or other electron beam device) depends on the number of beamlets, and the higher the number of beamlets, the higher the throughput. However, there are significant challenges in increasing the number of beamlets. For example, as the emission angle of the electron source increases, the global electron beam, and thus the angular intensity distribution of the beamlets, decreases in uniformity. Also, due to the increase in the field of view associated with the increase in the number of beamlets, field curvature, spherical aberration, and distortion increase.

Means for Solving the Problems

[0005] In some embodiments, the electron beam device includes a laser and a photocathode film. The photocathode film has a front side and a back side, and emits a plurality of electron beamlets when irradiated from the back side using a laser. The electron beam device also includes electrodes that extract a plurality of electron beamlets from the front side of the photocathode film and control the shape of the plurality of electron beamlets.

[0006] In some embodiments, a method of operating an electron beam device includes irradiating the back side of a photocathode film using a laser to cause the photocathode film to emit a plurality of electron beamlets. The method also includes extracting a plurality of electron beamlets from the front side of the photocathode film and directing the plurality of electron beamlets towards a target and inspecting the target.

[0007] To better understand the various embodiments described, it is necessary to refer to the following drawings in conjunction with the forms for carrying out the subsequent invention.

Brief Description of the Drawings

[0008]

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DETAILED DESCRIPTION OF THE INVENTION

[0009] Next, various embodiments are referred to in detail, and examples thereof are shown in the accompanying drawings. In the following detailed description, many specific details are set forth to provide a complete understanding of the various embodiments described. However, it will be apparent to those skilled in the art that the various embodiments described may be practiced without these specific details. In other instances, well-known methods, procedures, components, circuits, and networks are not described in detail so as not to unnecessarily obscure aspects of the embodiments.

[0010] In electron optics, the ranging action is provided by an electric field and / or a magnetic field. The components used to generate these fields may be referred to as lenses, as well as the fields themselves. A particular component may be part of multiple lenses. For example, a particular component may be both the final component of a first lens and the first component of a second lens.

[0011] FIG. 1 is a side cross-sectional view of a portion of an electron beam device 100 along an electron optical column, according to some embodiments. In some embodiments, the electron beam device 100 is a SEM used to inspect a target (e.g., a semiconductor wafer). The electron beam device 100 includes a photocathode film 102 disposed on a substrate 108. The photocathode film 102 is patterned to include an emission region 104 (e.g., a circular region having a diameter d that may be in the range of 10 to 30 μm) and a non-emission region 106. The emission region 104 is surrounded by the non-emission region 106 and is thinner than the non-emission region 106. For example, the emission region 104 has a substantially uniform first thickness (within manufacturing tolerances), and the non-emission region 106 has a substantially uniform second thickness that is thicker than the first thickness (within manufacturing tolerances).

[0012] The photocathode film 102 has a rear side facing the substrate 108 and a front side facing outward from the substrate 108. The photocathode film 102 is disposed on the front side of the substrate 108. The substrate 108 can be irradiated with light 110 (e.g., laser light) from the rear side of the substrate 108 (i.e., the side opposite to the side where the photocathode film 102 is disposed). The light 110 is supplied by a light source (e.g., the laser 501 in FIG. 5). The substrate 108 is transparent to the light 110, and as a result, by irradiating the substrate 108 with the light 110, the rear side of the photocathode film 102 is irradiated with the light 110. The light 110 irradiates both the emission region 104 and the non-emission region 106. This irradiation causes electrons to be emitted from the emission region 104 according to the photoelectric effect.

[0013] In some embodiments, the photocathode film 102 is gold. For example, the photocathode film 102 is a gold film having step layers. The film thickness of the emission region 104 can be in the range of 10 to 20 nm, and the film thickness of the non-emission region 106 can be at least five times the thickness of the layer of the emission region 104. The thickness of the non-emission region 106 prevents the emission of electrons from the non-emission region 106. The gold film is a useful photoelectron emitter due to its stability and well-characterized photoelectron emission properties. The work function of the gold film is approximately 4.2 eV, which is in good agreement with the available laser wavelengths. Since the maximum energy spread of the photoemitted electrons is the energy difference between the photon energy and the work function, by using an argon laser with a frequency twice as high to provide light 110 with a wavelength of 257 nm (4.8 eV), the energy spread can be maintained at a full width at half maximum of 0.6 eV or less.

[0014] Other examples of the photocathode film 102 include, but are not limited to, III-V semiconductors, cesium telluride (CsTe), and chalcogenides. In some embodiments, the photocathode film 102 is a material having a negative electron affinity. In some embodiments, the substrate 108 is sapphire.

[0015] The portion of the electron beam device 100 shown in FIG. 1 also has an electron gun optical system, which includes a Wehnelt electrode (Weh) 112, an extraction electrode (Ext) 114, and an anode 116. The Wehnelt electrode 112 is disposed between the front side of the photocathode film 102 and the extraction electrode 114. The extraction electrode 114 is disposed between the Wehnelt electrode 112 and the anode 116. The extraction electrode 114 extracts an electron beamlet 120 from the emission region 104 of the photocathode film 102 and directs the electron beamlet 120 into the acceleration region between the extraction electrode 114 and the anode 116. The electron beamlet 120 is composed of electrons emitted by the emission region 104 in response to irradiation of the photocathode film 102 with light 110. The Wehnelt electrode 112 controls the beam shape of the electron beamlet 120 after it is emitted from the emission region 104 of the photocathode film 102. The anode 116 accelerates the electron beamlet to a desired beam energy (BE). Each of the electrodes 112, 114, and 116 includes a respective bore aligned with the emission region 104 to permit passage of the electron beamlet 120 through the electrodes. The photocathode film 102, the Wehnelt electrode 112, the extraction electrode 114, and the anode 116 are each biased at a respective voltage V RS , V Weh , V Ext , and V BE . The anode may be grounded (i.e., V BE = 0).

[0016] In some embodiments, the light 110 is pulsed (e.g., by pulsing the laser 501 of FIG. 5), resulting in pulsing of the electron beamlet 120. In some embodiments, the light 110 is circularly polarized, resulting in deflection of the electron beamlet 120 (i.e., a wave such as deflection of electrons within the electron beamlet 120).

[0017] The beam limiting aperture (BLA) 118 is disposed behind the anode 116 along the z-axis (and along the path of the electron beamlet 120), such that the anode 116 is disposed between the extraction electrode 114 and the beam limiting aperture 118. The beam limiting aperture has a bore size (e.g., diameter) smaller than the bore size of the bore of the anode 116. The bore size of the beam limiting aperture 118 is selected to select the central portion of the electron beamlet 120, thereby allowing the passage of the central portion while blocking and discarding the non-central portion of the electron beamlet 120. In this way, the beam limiting aperture 118 controls the spread of the geometric aberration of the electron beamlet 120. The beam limiting aperture 118 is at the same voltage V as the anode 116 BE and may be biased (e.g., may be grounded).

[0018] The source of the electron beamlet 120 is actually the emission region 104 of the photocathode film 102 and can be modeled using a virtual source 124 (XO VS ) behind the substrate 108. In this modeling, an actual beam crossover XO of the electrons in the electron beamlet 120 created using the extraction electrode 114 RS is imaged. XO RS is the object of XO VS , and XO VS is on the image side of the anode 116 having the same potential V as the anode 116 with respect to the beam energy of the electron beamlet 120 BE . Thus, the virtual source trajectory 122 is a straight line and is the tangent to the actual source trajectory of the electrons of the electron beamlet 120 at the exit surface of the beam limiting aperture 118.

[0019] XO RS The magnification M from XO VS to XO is given by the following equation.

Equation

Equation

[0020] The magnification M of Equation (1) and the virtual source position L VS of Equation (2) can be estimated for high beam energy (e.g., 30 keV or higher). In this case, the coefficient ρ in Equation (1) is much larger than 1, which is because XO RS is near the Wehnelt electrode 112 and the extraction electrode 114, and the potential V XORS is much lower than V BE before the beam is accelerated. Therefore, Equation (1) and Equation (2) can be approximated by M≒2 / 3 = 0.67 and L VS ≒4×(L RS -z XORS ) / 3≒1.33×L RS respectively.

[0021] The portion of the photocathode film 102 shown in FIG. 1 includes only a single emission region 104 and emits a single electron beamlet 120. Similarly, each portion of the electrodes 112, 114, and 116 shown in FIG. 1 has only a corresponding single bore, and only a single beam-limiting aperture 118 is shown. However, the photocathode film 102 may include a plurality of emission regions 104 separated from each other by non-emission regions 106. Each of the emission regions 104 emits its respective electron beamlet 120 in response to irradiation of the rear side of the photocathode film 102 by light 110. No electron beamlets are emitted from the non-emission regions 106 due to the thickness of the non-emission regions 106. The resulting plurality of electron beamlets 120 are extracted from the emission regions 104 using the extraction electrode 114, shaped using the Wehnelt electrode 112, and accelerated using the anode 116. The electrodes 112, 114, and 116 each include a plurality of bores corresponding to the plurality of emission regions 104 and the plurality of electron beamlets 120. Each bore is aligned with the respective emission region 104 of its respective electron beamlet 120. The beam-limiting aperture 118 shown in FIG. 1 is one of the plurality of beam-limiting apertures 118, selects the central region of the plurality of electron beamlets 120, and blocks the non-central regions. Each of the plurality of beam-limiting apertures 118 of the plurality of beam-limiting apertures 118 selects the central region of its respective electron beamlet 120 of the plurality of electron beamlets 120 and blocks the non-central regions. Each beam-limiting aperture 118 is aligned with its respective emission region 104 and the respective bores of the electrodes 112, 114, and 116.

[0022] In some embodiments, each of the emission regions 104 within the photocathode film has the same diameter d (e.g., within manufacturing tolerances). The emission regions 104 may be arranged in an array and may be equally spaced (e.g., rotationally symmetric). The bores of the electrodes 112, 114, and 116 may have the same arrangement as the plurality of beam-limiting apertures 118.

[0023] FIG. 2 is a plan view of a patterned photocathode film 200 for use in an electron beam device according to some embodiments. The photocathode film 200 is an example of the photocathode film 102 (FIG. 1). The photocathode film 200 includes a plurality of emission regions 204 surrounded by non-emission regions 202. The emission regions 204 are an example of the emission regions 104, and the non-emission regions 202 are an example of the non-emission regions 106 (FIG. 1). The emission regions 204 are arranged in a hexagonal array in the x-y plane perpendicular to the z-axis of FIG. 1. The hexagonal array is desirable because the hexagon has relatively high rotational symmetry. Alternatively, the emission regions 204 may be arranged in different patterns (e.g., with different rotational symmetries). In some embodiments, the emission regions 204 are circular in the x-y plane, the diameter d ranges from 10 to 30 μm, and the spacing (i.e., pitch) s ranges from 75 to 150 μm.

[0024] The number of emission regions 204 determines and is equal to the number of electron beamlets 120 emitted from the photocathode film 200. In the case of the hexagonal pattern of FIG. 2, the total number of electron beamlets 120, MEB tot is as follows.

Equation

[0025] The photocathode film 200 can be fabricated by forming a thick film (e.g., a thick gold film) on a substrate (e.g., a sapphire substrate), defining holes with diameter d and spacing s in the film (e.g., down to the substrate) by photolithography and etching, and then forming a thin film (e.g., a thin film of the same material as the thick film, e.g., a thin gold film) on the thick film and within the holes. The photocathode film 200 is irradiated (e.g., from the rear side using light 110 in FIG. 1), and as a result, electrons are emitted from the emission region 204, but no electrons are emitted from the thicker non-emission region 202.

[0026] The Wehnelt electrode 112, extraction electrode 114, anode 116, and plurality of beam-limiting apertures 118 can be formed using an array of bores (i.e., apertures) having the same arrangement as the arrangement of the emission region 204 in the photocathode film 200. The bores have the same spacing s as the emission region 204 and are aligned with the emission region 204. The bores of the Wehnelt electrode 112, extraction electrode 114, and anode 116 have a diameter larger than the diameter of the emission region 204. The bores of the plurality of beam-limiting apertures (i.e., apertures) have a diameter smaller than the diameter of the bores of the Wehnelt electrode 112, extraction electrode 114, and anode 116 (i.e., the bore size of the plurality of beam-limiting apertures is smaller than the bore size(s) of the Wehnelt electrode 112, extraction electrode 114, and anode 116). Thus, instead of showing the photocathode film 200, FIG. 2 can be considered to show a conductive (e.g., metal) plate for use as the Wehnelt electrode 112, extraction electrode 114, anode 116, or plurality of beam-limiting apertures 118.

[0027] Figure 3 is a side cross-sectional view of a portion of an electron beam device 300 along an electron optical column, according to some embodiments. The electron beam device 300 includes a photocathode film 302, a Wehnelt electrode 312, and an extraction electrode 314, which may be examples of the photocathode film 102, the Wehnelt electrode 112, and the extraction electrode 114 (FIG. 1). Alternatively, the photocathode film 302 may be an example of the photocathode film 906 (FIG. 9) (e.g., with the Wehnelt electrode 312 and the extraction electrode 314 being examples of the Wehnelt electrode 112 and the extraction electrode 114). The electron beam device 300 may further include an anode (e.g., the anode 116 of FIG. 1) and a plurality of beam limiting apertures (including, for example, the beam limiting aperture 118 of FIG. 1) located to the right of the portion of the electron beam device 300 shown in FIG. 3. The photocathode film 302 may be an example of the photocathode film 200 (FIG. 2) in which the Wehnelt electrode 312, the extraction electrode 314, the anode, and the plurality of beam limiting apertures are configured accordingly.

[0028] Figure 3 shows computer-simulated ray trajectories of electrons within electron beamlets 320 (including the central electron beamlet 320-1) emitted from respective emission regions 304 of the photocathode film 302. The emission regions 304 (including the central emission region 304-1) may be examples of the emission region 104 (FIG. 1) or the region irradiated by the optical beamlets 904 (FIG. 9). The trajectories are caused by biasing the photocathode film 302, the Wehnelt electrode 312, the extraction electrode 314, and the anode at respective voltages V RS , V Weh , V Ext , and V BE . In some embodiments, the emission regions 304 have a diameter in the range of 10 - 30 μm, and the bores of the Wehnelt electrode 312, the extraction electrode 314, and the anode have a diameter in the range of 50 - 150 μm.

[0029] In some embodiments, the Wehnelt electrode 312, extraction electrode 314, and / or anode have outer bores that are not for use with any of the electron beamlets 320 and do not correspond to any of the emission regions 304. These outer bores include an outer bore 324 for the Wehnelt electrode 312, an outer bore 326 for the extraction electrode 314, and an outer bore for the anode that can be aligned with the outer bore 324 and the outer bore 326. These outer bores are dummy bores (i.e., dummy holes) that enhance the uniformity of the electric field around the bores through which the electron beamlets 320 pass. For example, the dummy bores surround the bores through which the electron beamlets 320 pass. In the example of FIG. 2, FIG. 2 shows a plate for use as a Wehnelt electrode 112, extraction electrode 114, or anode 116, and one or more of the outer rings of hexagonal bores can be dummy bores. The dummy bores reduce the difference in electron trajectories between the inner and outer electron beamlets 320 (e.g., ensuring that the difference is negligible).

[0030] FIG. 4 is an enlarged view of the central electron beamlet 320-1 of the electron beam device 300 (FIG. 3) according to some embodiments. The central electron beamlet 320-1 is emitted from the central emission region 304-1 of the photocathode film 302. The electron trajectory and optical characteristics of the central electron beamlet 320-1 can be characterized by the central electron line 402 and the edge electron line 404, and the actual source crossover XO RS and the real image plane 406 are formed and shown.

[0031] Returning to FIG. 2, in one example, the photocathode film 200 has 331 emission regions 204 distributed in hexagons (i.e., 10 rings, M x = 21). The emission regions 204 can have a diameter of 20 μm and a spacing of 100 μm. All of the emission regions 204 have a diameter D iIt can be irradiated by a laser beam with a wavelength of 2000 μm. According to the experimental quantum efficiency measurement using a gold film with a thickness of 10 - 20 nm, a 2 W, 257 nm laser can supply an electron beamlet current of 1 nA from each emission region 204, and the total current from all 331 emission regions 204 is 331 nA. The laser power can be adjusted from 1 W to 10 W, so the current of each electron beamlet can be adjusted from 0.5 nA to 5 nA, and the total current from all 331 emission regions 204 is 165 nA - 1650 nA. Such electron beamlets and the corresponding beamlet currents can provide high - throughput inspection and review of targets (e.g., wafers) in SEM.

[0032] Other examples are also possible where the laser power is adjusted according to the currents of multiple electron beamlets. The currents of multiple electron beamlets can be adjusted while adjusting the laser power, without changing the settings of the electron optical system in the vacuum chamber 130 and without recalibrating the electron optical column.

[0033] An optical lens can be used to expand the beam size of the laser beam and irradiate the array of emission regions 104 on the photocathode film 102 (Figure 1). Figure 5 shows an optical system 500 including a pair of lenses 504 and 506 arranged in series along the z - axis and expanding the beam size of the laser beam 502 according to some embodiments. The laser 501 (e.g., having an adjustable laser power) generates the laser beam 502, which is an example of the light 110 (Figure 1). Thus, the pair of lenses 504 and 506 may be part of the electron beam device 100 (Figure 1) (e.g., the electron beam device 300 in Figures 3 - 4). As the output of the laser, the laser beam 502 has a beam size D 0 After passing through the first lens 504 and the second lens 506, the laser beam 502 has an expanded beam size D sufficient to irradiate the emission regions 104 on the photocathode film 102 (e.g., all of the emission regions 204 on the photocathode film 200 in Figure 2). i (e.g., D ihas a size of 2000 μm or more). The first lens 504 focuses the laser beam 502 onto the crossover XO, and the second lens 506 collimates the laser beam 502. The resulting optical magnification is D i / D 0 =f 2 / f 1 where f 1 is the focal length of the first lens 504 and f 2 is the focal length of the second lens 506. In some embodiments, the optical magnification is in the range of 5 to 10 times (i.e., a factor of 5 to 10). The enlarged laser beam 502 having the enlarged beam size D i irradiates the rear side of the photocathode film 102 through the substrate 108, induces photoemission in the emission region 104 of the photocathode film 102, and causes the emission of electron beamlets 120 from the emission region 104. The optical magnification of the laser beam 502 does not change the power or wavelength of the laser beam 502.

[0034] In some embodiments, the electron beam device 100 (FIG. 1) (e.g., the electron beam device 300 of FIGS. 3-4) includes image ranging that focuses a plurality of electron beamlets onto an intermediate image plane. FIG. 6 is a side cross-sectional view along the electron optical column of an electron beam device 600 (e.g., the electron beam device 100 of FIG. 1, the electron beam device 300 of FIGS. 3-4), and the image ranging includes an image lens array (ILA) 602 disposed between the anode 116 and the intermediate image plane 604 (and thus between the plurality of beam limiting apertures 118 and the intermediate image plane 604) that focuses the plurality of electron beamlets 606 onto the intermediate image plane 604. The image lens array 602 includes respective lenses that focus respective electron beamlets 606 onto the intermediate image plane 604. For example, the image lens array 602 includes image lenses for each electron beamlet 606 of the plurality of electron beamlets 606 and focuses the electron beamlets 606 onto the intermediate image plane 604.

[0035] The lenses of the image lens array 602 can be electrostatic lenses or magnetic lenses. For example, the lens may be an Einzel lens, which is an electrostatic lens with three electrode plates, namely the left plate 624 closest to the anode 116, the right plate 628 closest to the intermediate image plane 604, and the intermediate plate 626 between the left plate 624 and the right plate 628. In some embodiments, the three electrode plates 624, 626, and 628 are mounted with bores distributed in a hexagonal pattern according to FIG. 2. The bore size may be equal to or approximately equal to the bore size of the bore of the anode 116. The left plate 624 and the right plate 628 may be grounded, and the intermediate plate 626 may be biased with a voltage V IL to focus the electron beamlets 606.

[0036] FIG. 7 is a side cross-sectional view along the electron optical column of an electron beam device 700 (e.g., the electron beam device 100 of FIG. 1, the electron beam device 300 of FIGS. 3-4), where image ranging includes a global image lens (IL) 702 disposed between the anode 116 and the intermediate image plane 704 (and thus between the plurality of beam limiting apertures 118 and the intermediate image plane 704), focusing a plurality of electron beamlets 706 onto the intermediate image plane 704, thereby forming an image of the plurality of electron beamlets 706 within the intermediate image plane 704. The global image lens 702 has a single bore for the plurality of electron beamlets 706 and forms a crossover 708 of the plurality of electron beamlets 706. The crossover 708 is located between the global image lens 702 and the intermediate image plane 704. The global image lens 702 can be an electrostatic lens or a magnetic lens. The magnetic global image lens allows for a large reduction in the upper column because the image distance can be shortened without causing high voltage problems.

[0037] The electron beam device 700 also includes a field lens (FL) 710 that collimates a plurality of electron beamlets 706 after the crossover 708. The field lens 710 is a global lens having a single bore for the plurality of electron beamlets 706. The field lens 710 coincides with the intermediate image plane 704. For example, the intermediate image plane 704 is within the principal plane of the field lens 710. The field lens 710 may be an electrostatic lens or a magnetic lens.

[0038] The electron optical columns of the electron beam device 600 (FIG. 6) and the electron beam device 700 (FIG. 7) include an upper column and a lower column. The upper column can be considered to extend from the virtual source plane 608 where the virtual source 124 is located (or from the substrate 108) to the intermediate image plane 704. The lower column can be considered to extend from the intermediate image plane 704 to the target 622 (e.g., a semiconductor wafer) to be inspected (or to the stage on which the target 622 is mounted).

[0039] The lower columns of the electron beam apparatuses 600 and 700 include a transfer lens (TL) 610 and an objective lens (OL) 620. The transfer lens 610 is disposed on the opposite side of the intermediate image plane 604 or 704 from the image ranging (e.g., the image lens array 602 of FIG. 6, or the global image lens 702 of FIG. 7). The transfer lens 610 forms a crossover 618 of a plurality of electron beamlets 606 or 706. The crossover 618 is located between the transfer lens 610 and the objective lens 620. In some embodiments, the crossover 618 is closer to the objective lens 620 than the transfer lens 610, resulting in a reduction of the plurality of electron beamlets 606 or 706. The objective lens 620 focuses the plurality of electron beamlets 606 or 706 onto the target 622 and also decelerates the plurality of electron beamlets 606 or 706 to a desired landing energy. The objective lens 620 is disposed between the crossover 618 and the target 622 (and thus between the crossover 618 and the stage for the target 622). The transfer lens 610 is disposed between the intermediate image plane 604 or 704 and the crossover 618. In the electron beam apparatus 700, the transfer lens 610 is disposed between the field lens 710 and the objective lens 620. The transfer lens 610 may be an electrostatic lens or a magnetic lens. The objective lens 620 may be an electrostatic lens or a mixed magnetic-electrostatic lens. The transfer lens 610 and the objective lens 620 are global lenses each having a single bore (i.e., where the plurality of electron beamlets 606 or 706 pass through) for the plurality of electron beamlets 606 or 706.

[0040] The plurality of electron beamlets 606 or 706 incident on the transfer lens 610 are collimated, such that the electron beamlets 606 or 706 irradiate the transfer lens 610 telecentrically. In the example of the electron beam apparatus 700 (FIG. 7), the field lens 710 collimates the plurality of electron beamlets 706 to cause the plurality of electron beamlets 706 to irradiate the transfer lens 610 telecentrically.

[0041] The electron optical systems of the lower columns of the electron beam devices 600 and 700 are projection optical systems. The projection optical system can have a large optical reduction (e.g., ~10×) using a crossover 618 located near the objective lens 620, and performs both reducing the influence of Coulomb interaction on the resolution and reducing the off-axis aberration of the outer ring electron beamlets 606 or 706. The optical performance (i.e., the resolution of individual electron beamlets 606 or 706) is mainly dominated by the Coulomb interaction around the crossover 618. Using a pure electrostatic objective lens 620 significantly reduces the influence of Coulomb interaction, and accordingly improves the resolution. For a relatively low beam current, the influence of the Coulomb interaction around the crossover 618 is reduced, but the source image may reduce the resolution because the virtual source size of the plurality of emission regions 104 is large.

[0042] According to computer simulations, the actual source crossover (XO RS ) size can be about 300 nm for a 20 - 80% current increase measurement. The size of the virtual source 124 (XO VS ) may be about 0.67×300 nm ≒ 200 nm. The optical reduction of the lower column of the electron beam device 600 or 700 can be about 10× when using a pure electrostatic objective lens 620. The optical reduction of the upper column of the electron beam device 700 (Figure 7) may be greater than 5×, and provides a source image at the target 622 better than 200 / (10×5) = 4 nm for a relatively low beam current (e.g., a single electron beamlet current of less than 1 nA). For a higher beam current, the blur due to the Coulomb interaction around the crossover 618 may become dominant over the entire source image. The Coulomb interaction between the electron beamlets 606 or 706 in the upper column can be ignored because the spacing between the electron beamlets (e.g., 100 μm or more) is much larger than the average spacing of electrons within a single electron beamlet 606 or 706.

[0043] The image distance of the image lens array 602 (FIG. 6) or the global image lens 702 (e.g., the distance from the virtual source 124 to the intermediate image plane 604 or 704) can be selected to reduce the focusing voltage and avoid an overly high breakdown voltage across the gap of the image lens array 602 (FIG. 6) or the global image lens 702 (FIG. 7). For example, the image distance can be 50 mm or more, and the length of the upper column from the virtual source plane 608 to the intermediate image plane 604 or 704 can be at least 300 mm.

[0044] In addition to the transfer lens 610 and the objective lens 620, the lower columns of the electron beam devices 600 and 700 also include a first Wien filter 612 and a second Wien filter 616, both of which are disposed between the transfer lens 610 and the objective lens 620. The second Wien filter 616 is closer to the objective lens 620 than the first Wien filter 612, and the first Wien filter 612 is closer to the transfer lens 610 than the second Wien filter 616. Thus, the first Wien filter 612 is disposed between the transfer lens 610 and the second Wien filter 616, and the second Wien filter 616 is disposed between the first Wien filter 612 and the objective lens 620.

[0045] The second Wien filter 616 deflects secondary electrons 630 from the target 622 to the detector 614 (e.g., detector array), and the detector 614 detects the secondary electrons 630. The secondary electrons 630 are electrons from a plurality of electron beamlets 606 or 706 scattered from the target 622. By deflecting the secondary electrons 630 from the target 622 to the detector 614, the second Wien filter 616 collects the secondary electrons 630 and enables inspection of the target 622.

[0046] The second Wien filter 616 is strong enough to direct secondary electrons 630 having a deflection angle wide enough to properly image the target 622 towards the detector 614. Such a powerful Wien filter generates an energy spread because of the spread of the source energy (e.g., 0.6 eV or less as described above). This energy spread causes blurring of the plurality of electron beamlets 606 or 706. The first Wien filter 612 compensates the second Wien filter 616 by correcting this energy spread, and the energy spread generated by the first Wien filter 612 compensates the energy spread generated by the second Wien filter 616, thus reducing or removing the blurring.

[0047] FIG. 8 is a cross-sectional view of a magnetic lens stack 800 with magnetic lenses sharing magnetic pole pieces 802 according to some embodiments. The magnetic lenses in the magnetic lens stack 800 include a first image lens 804, a second image lens 806, a field lens 808, and a transfer lens 810. The first image lens 804 and the second image lens 806 constitute an example of the global image lens 702 (FIG. 7). The field lens 808 and the transfer lens 810 are examples of the field lens 710 and the transfer lens 610 (FIG. 7), respectively. The magnetic lens stack 800 with the shared magnetic pole pieces 802 enables precise control of mechanical tolerances. In addition to the beam focusing function described for the electron beam device 700 (FIG. 7), the magnetic lens stack 800 can be used to correct the image rotation of the plurality of electron beamlets 706 by applying reverse currents (i.e., currents in the opposite direction) to the different coils of the respective lenses 804, 806, 808, and 810.

[0048] FIG. 9 shows an optical system 900 for an electron beam device according to some embodiments, in which the patterned photocathode film 102 is replaced by a photocathode film 906, and the photocathode film 906 emits a plurality of electron beamlets when irradiated from the rear side by a plurality of optical beamlets 904. The photocathode film 906 is disposed on a substrate 108. The photocathode film 906 may be unpatterned (e.g., flat) and is thin enough (e.g., having a thickness of 10 to 20 nm) to enable photoemission in response to irradiation by the plurality of optical beamlets 904. The plurality of optical beamlets 904 irradiate respective regions 908 of the photocathode film 906. The plurality of electron beamlets (e.g., the electron beamlets 320 of FIG. 3) are emitted from the respective regions 908 when the respective regions 908 are irradiated by the plurality of optical beamlets 904. The electron beamlets are not emitted from regions of the photocathode film 906 that are not irradiated. In some embodiments, the photocathode film 906 is gold. Other examples of the photocathode film 906 include, but are not limited to, III-V semiconductors, cesium telluride (CsTe), and chalcogenides. In some embodiments, the photocathode film 906 is a material having a negative electron affinity.

[0049] The optical system 900 includes a laser 501 (FIG. 5) that generates a laser beam 502, and may include a pair of lenses 504 and 506 (FIG. 5) that expand the laser beam 502. The optical aperture array 902 divides the laser beam 502 (e.g., expanded by a pair of lenses 504 and 506) into a plurality of light beamlets 904. The optical aperture array (LAA) 902 is disposed between the laser 501 and the rear side of the photocathode film 906 (and between the laser 501 and the substrate 108). (The rear side of the photocathode film 906 is the side facing the substrate 108.) For example, a pair of lenses 504 and 506 are disposed between the laser 501 and the optical aperture array 902, and the optical aperture array 902 is disposed between the second lens 506 and the substrate 108. In some embodiments, the optical aperture array 902 is formed on the substrate 108. Since the plurality of light beamlets 904 that irradiate the photocathode film 906 are emitted from the laser 501, the photocathode film 906 is irradiated using the laser 501. The optical aperture array 902 is an array of apertures, and in some embodiments, the apertures of the optical aperture array 902 are arranged in a hexagonal shape as shown in FIG. 2. In some embodiments, the bore size (e.g., diameter) of the apertures of the optical aperture array 902 is in the range of 10 to 30 μm.

[0050] In some embodiments, the laser 501 is pulsed, resulting in the pulsing of the laser beam 502, the plurality of light beamlets 904, and the plurality of electron beamlets emitted from the photocathode film 906. In some embodiments, the laser beam 502 is circularly deflected, resulting in the circular deflection of the plurality of light beamlets 904 and the corresponding deflection of the plurality of electron beamlets emitted from the region 908 of the photocathode film 906.

[0051] The optical system 900 and the photocathode film 906 can be combined with the electron optical column of an electron beam device 100 (FIG. 1) (e.g., the electron beam device 300 of FIGS. 3 to 4). For example, the optical system 900 and the photocathode film 906 can be combined with the electron optical column of an electron beam device 600 (FIG. 6) or 700 (FIG. 7).

[0052] As described above, the photocathode film 102 or 906 is disposed on the front side of the substrate 108. In some embodiments, the substrate 108 is incorporated into the vacuum chamber 130 of the electron optical column. By facing the front side of the substrate 108 into the optical vacuum chamber 130, the photocathode film 102 or 906 is located within the vacuum chamber 130. The rear side of the substrate 108 is irradiated using a laser (e.g., laser 501 of FIG. 5 or FIG. 9) and exposed to air. Thus, the optical system of the electron beam device may be in air, but the electron optical system is in vacuum.

[0053] FIG. 10 is a flowchart showing a method 1000 of operating an electron beam device (e.g., electron beam device 100 of FIG. 1, electron beam device 300 of FIGS. 3 - 4, electron beam device 600 of FIG. 6, electron beam device 700 of FIG. 7) according to some embodiments. In method 1000, the rear side of the photocathode film is irradiated using a light source that can be a laser (e.g., laser 501 of FIG. 5 or FIG. 9) to emit a plurality of electron beamlets from the photocathode film (1002). The light irradiating the photocathode film may be pulsed and / or circularly polarized, causing pulsing and / or deflection of the plurality of electron beamlets.

[0054] In some embodiments, the photocathode film (e.g., photocathode film 102 of FIG. 1, photocathode film 200 of FIG. 2) includes a plurality of emission regions (e.g., emission region 104 of FIG. 1, emission region 204 of FIG. 2) separated from each other by non - emission regions (e.g., non - emission region 106 of FIG. 1, non - emission region 202 of FIG. 2) (1004). The plurality of emission regions are thinner than the non - emission regions. By irradiating the rear side of the photocathode film, a plurality of electron beamlets are emitted from the emission regions of the photocathode film.

[0055] In some other embodiments, a laser beam (e.g., the laser beam 502 in FIG. 9) is generated (1006) using a laser and split into a plurality of light beamlets (e.g., the light beamlets 904 in FIG. 9) using an optical aperture array (e.g., the optical aperture array 902 in FIG. 9). Each region (e.g., the region 908 in FIG. 9) of the photocathode film (e.g., the photocathode film 906 in FIG. 9) is illuminated (1006) from the rear side of the photocathode film using the plurality of light beamlets.

[0056] A plurality of electron beamlets are extracted (1008) from the front side of the photocathode film. The plurality of electron beamlets can be extracted using extraction electrodes (e.g., the extraction electrode 114 in FIGS. 1, 6, and / or 7, the extraction electrode 314 in FIGS. 3 - 4).

[0057] In some embodiments, the plurality of electron beamlets are extracted (1010) from the emission region in step 1004 (e.g., the emission region 104 in FIG. 1, the emission region 204 in FIG. 2). In some other embodiments, the plurality of electron beamlets are extracted (1012) from each region (e.g., the region 908 in FIG. 9) irradiated with the plurality of light beamlets in step 1006.

[0058] Using a plurality of beam - limiting apertures (e.g., the plurality of beam - limiting apertures 118 in FIGS. 1, 6, and / or 7), the central portions of the plurality of electron beamlets can be selected and the non - central portions of the plurality of electron beamlets can be blocked (1014).

[0059] A plurality of electron beamlets are directed (1016) at a target (e.g., target 622 of FIGS. 6 or 7) to inspect the target. In some embodiments, the target is a semiconductor wafer. Directing the plurality of electron beamlets may include controlling the shape of the plurality of electron beamlets using a Wehnelt electrode (e.g., Wehnelt electrode 112 of FIGS. 1, 6, and / or 7, Wehnelt electrode 312 of FIGS. 3 - 4), and accelerating the plurality of electron beamlets using an anode (e.g., anode 116 of FIGS. 1, 6, and / or 7). The Wehnelt electrode is disposed between the front side of the photocathode film and the extraction electrode. The extraction electrode is disposed between the Wehnelt electrode and the anode. The anode is disposed between the extraction electrode and a plurality of beam limiting apertures and may have an opening for the plurality of electron beamlets having a bore size larger than the bore size of the plurality of beam limiting apertures.

[0060] In some embodiments, as part of directing the plurality of electron beamlets at the target, the plurality of electron beamlets are focused (1018) onto an intermediate image plane (e.g., intermediate image plane 604 or 704 of FIGS. 6 or 7). In some embodiments, this focusing is performed using an image lens array (e.g., image lens array 602 of FIG. 6) disposed between the anode and the intermediate image plane. In some other embodiments, this focusing is performed using a global image lens (e.g., global image lens 702 of FIG. 7) disposed between the anode and the intermediate image plane. Focusing performed using the global image lens includes forming a crossover (e.g., crossover 708 of FIG. 7) of the plurality of electron beamlets between the global image lens and the intermediate image plane, and directing the plurality of electron beamlets at the target further includes focusing and collimating the plurality of electron beamlets onto the intermediate image plane. Collimation may be performed using a field lens (e.g., field lens 710 of FIG. 7).

[0061] The crossover of a plurality of electron beamlets (e.g., the crossover 618 in FIGS. 6 or 7) is located on the side of the intermediate image plane opposite to the anode and is formed (1020) using a transfer lens (e.g., the transfer lens 610 in FIGS. 6 or 7). (This crossover may be different from the crossover formed by the global image lens.) The plurality of electron beamlets are focused and decelerated (1022) onto the target using an objective lens (e.g., the objective lens 620 in FIGS. 6 or 7). The objective lens is disposed between this crossover and the target.

[0062] Method 1000 may further include using first and second Wien filters. The second Wien filter (e.g., the second Wien filter 616 in FIGS. 6 or 7) is closer to the objective lens than the first Wien filter (e.g., the first Wien filter 612 in FIGS. 6 or 7) and deflects secondary electrons from the target to the detector (e.g., the detector 614 in FIGS. 6 or 7). The first Wien filter compensates for the influence of the second Wien filter on the plurality of electron beamlets.

[0063] In some embodiments of method 1000, the photocathode film is disposed on the front side of a substrate (e.g., the substrate 108 in FIGS. 1, 6, 7, and / or 9) incorporated in the electron optical column. The photocathode film disposed on the front side of the substrate is exposed to the vacuum (e.g., within the vacuum chamber 130 in FIGS. 1, 3 - 7, and / or 9) within the electron optical column. The rear side of the substrate is exposed to air (e.g., the air 132 in FIGS. 1, 3 - 7, and / or 9).

[0064] Multiple iterations of method 1000 may be performed. For example, different iterations of method 1000 may be performed to inspect different targets and / or to repeatedly inspect the same target. The laser power may be adjusted during the iterations. Thus, different laser powers, i.e., different electron beamlet currents corresponding to different laser powers, may be used to inspect different targets and / or to repeatedly inspect the same target. For example, a first iteration of method 1000 may be performed using a laser configured to have a first laser power for a first target. After the first iteration is completed, the laser may then be reconfigured to have a second laser power that is different from (e.g., greater than or less than) the first laser power. Next, a second iteration of method 1000 may be performed using the laser reconfigured to have the second laser power for a second target. Alternatively, the second iteration may be performed using the laser reconfigured to have the second laser power for the first target. In some embodiments, the settings of the electron optical components in the vacuum of the electron optical column are not changed between iterations and the electron optical column is not recalibrated between iterations. Instead, the laser is reconfigured to change the laser power without adjusting the settings of the electron optical components in the vacuum and without recalibrating the electron optical column.

[0065] The operations of method 1000 appear to be performed in a particular order in FIG. 10, but they may all be performed simultaneously (e.g., continuously). Method 1000 may include more or fewer operations than those shown. Two or more operations may be combined into one operation.

[0066] The foregoing description has been presented for purposes of illustration and description, and has been made with reference to particular embodiments. However, the above exemplary discussion is not intended to be exhaustive or to limit the claims to the precise forms disclosed. Many modifications and variations are possible in light of the above disclosure. The embodiments were chosen and described in order to best explain the principles underlying the claims and their practical application, thereby enabling those skilled in the art to best utilize the embodiments with various modifications as are suited to the particular use contemplated.

Claims

1. Lasers and, A photocathode film having a front side and a rear side, wherein when irradiated from the rear side using the laser, the photocathode film emits a plurality of electron beamlets, An electrode that extracts the plurality of electron beamlets from the front side of the photocathode film and controls the shape of the plurality of electron beamlets, Equipped with, The electrode is A Wehnert electrode having a first plurality of apertures for the plurality of electron beamlets, The extraction electrode having a second plurality of apertures for the plurality of electron beamlets, The anode includes a third anode having a plurality of apertures for the plurality of electron beamlets, The Wehnert electrode is positioned between the front side of the photocathode film and the extraction electrode, and The extraction electrode is positioned between the Wehnelt electrode and the anode. Electron beam device.

2. The photocathode film includes a plurality of emission regions separated from each other by non-emission regions. The plurality of emission regions are thinner than the non-emission regions, and The plurality of electron beamlets are emitted from the plurality of emission regions when the rear side of the photocathode film is irradiated using the laser, The electron beam apparatus according to claim 1.

3. The aforementioned photocathode film is made of gold, and The plurality of emission regions have a first thickness of 10 to 20 nm. The electron beam apparatus according to claim 2.

4. The electron beam apparatus according to claim 3, wherein the non-emitting region has a second thickness that is at least five times the first thickness.

5. An optical aperture array, further comprising an optical aperture array positioned between the laser and the rear side of the photocathode film, which divides the laser beam from the laser into a plurality of optical beamlets to irradiate each region of the photocathode film from the rear side thereof, Here, the plurality of electron beamlets are emitted from each of the regions when each of the regions is irradiated by the plurality of light beamlets. The electron beam apparatus according to claim 1.

6. The electron beam apparatus according to claim 5, further comprising a pair of lenses, the pair of lenses being positioned between the laser and the optical aperture array to magnify the laser beam.

7. The system further comprises a plurality of beam limiting apertures for selecting the central portion of the plurality of electron beamlets and blocking the non-central portion of the plurality of electron beamlets, where, The anode is positioned between the extraction electrode and the plurality of beam limiting apertures, and The bore size of the third plurality of apertures of the anode is larger than the bore size of the plurality of beam limiting apertures. The electron beam apparatus according to claim 1.

8. The electron beam apparatus according to claim 7, further comprising image ranging for focusing the plurality of electron beamlets onto an intermediate image plane.

9. The electron beam apparatus according to claim 8, wherein the image ranging includes an image lens array disposed between the anode and the intermediate image plane to focus the plurality of electron beamlets onto the intermediate image plane.

10. The image ranging is performed by an electron beam apparatus including a global image lens, which is positioned between the anode and the intermediate image plane to form a crossover of the plurality of electron beamlets and, after the crossover, focuses the plurality of electron beamlets onto the intermediate image plane, where the crossover is located between the global image lens and the intermediate image plane. The electron beam apparatus further includes a field lens that coincides with the intermediate image plane to collimate the plurality of electron beamlets. The electron beam apparatus according to claim 8.

11. A transfer lens is positioned on the opposite side of the intermediate image plane from the image ranging, and forms a crossover of the plurality of electron beamlets. The system further comprises an objective lens for focusing the plurality of electron beamlets onto a target, Here, the crossover is located between the transfer lens and the objective lens. The electron beam apparatus according to claim 8.

12. The system further comprises a first Wien filter and a second Wien filter disposed between the transfer lens and the objective lens, where, The second Wien filter is closer to the objective lens than the first Wien filter. The second Wien filter deflects secondary electrons from the target to the detector. The first Wien filter compensates for the second Wien filter. The electron beam apparatus according to claim 11.

13. The aforementioned photocathode film is positioned on the front side of the substrate. The substrate is incorporated into an electron-optical column including a vacuum chamber. The front side of the substrate faces the inside of the vacuum chamber, and the photocathode film is placed inside the vacuum chamber. The rear side of the substrate is irradiated using the laser to irradiate the rear side of the photocathode film, and is exposed to air. The electron beam apparatus according to claim 1.

14. The electron beam apparatus according to claim 1, wherein the laser has adjustable laser power to change the current of the plurality of electron beamlets.

15. A laser is used to irradiate the back side of the photocathode film, causing the photocathode film to emit multiple electron beamlets. Using extraction electrodes, the plurality of electron beamlets are extracted from the front side of the photocathode film, and, To direct the aforementioned plurality of electron beamlets toward the target and inspect the target, A method for operating an electron beam apparatus, including, The aforementioned direction The shapes of the plurality of electron beamlets are controlled using Wehnelt electrodes, and The plurality of electron beamlets are accelerated using an anode, wherein, The Wehnert electrode is positioned between the front side of the photocathode film and the extraction electrode, and The extraction electrode is positioned between the Wehnelt electrode and the anode. method.

16. The photocathode film includes a plurality of emission regions separated from each other by non-emission regions. The plurality of emission regions are thinner than the non-emission regions. The aforementioned irradiation causes the photocathode film to emit the plurality of electron beamlets from the emission region, and, The extraction includes extracting the plurality of electron beamlets from the emission region. The method according to claim 15.

17. The aforementioned irradiation is A laser beam is generated using the aforementioned laser, The laser beam is divided into multiple optical beamlets using an optical aperture array, and This includes irradiating each region of the photocathode film from the rear side of the photocathode film using the plurality of light beamlets, The extraction includes extracting the plurality of electron beamlets from each of the regions. The method according to claim 15.

18. The method further includes using multiple beam limiting apertures to select the central portion of the multiple electron beamlets and blocking the non-central portions of the multiple electron beamlets, wherein The anode is positioned between the extraction electrode and the plurality of beam limiting apertures, The bore size of the anode aperture for the plurality of electron beamlets is larger than the bore size of the plurality of beam limiting apertures. The method according to claim 15.

19. The method according to claim 15, further comprising directing the plurality of electron beamlets onto an intermediate image plane.

20. The method according to claim 19, wherein focusing the plurality of electron beamlets onto the intermediate image plane is performed using an image lens array positioned between the anode and the intermediate image plane.

21. Focusing the plurality of electron beamlets onto the intermediate image plane is performed using a global image lens positioned between the anode and the intermediate image plane. Focusing the plurality of electron beamlets onto the intermediate image plane includes forming a crossover of the plurality of electron beamlets between the global image lens and the intermediate image plane, and The orienting further includes collimating the plurality of electron beamlets focused on the intermediate image plane using a field lens. The method according to claim 19.

22. The aforementioned direction is The crossover of the plurality of electron beamlets is formed using a transfer lens, wherein the crossover is located on the opposite side of the intermediate image plane from the anode. The objective lens is used to focus and decelerate the plurality of electron beamlets onto the target, wherein the objective lens is positioned between the crossover and the target. The method according to claim 15, further comprising:

23. The method involves using a first Wien filter to compensate for the effect of a second Wien filter on the plurality of electron beamlets, wherein the second Wien filter is closer to the objective lens than the first Wien filter. The secondary electrons are deflected from the target to the detector using the second Wien filter, The method according to claim 22, further comprising:

24. The aforementioned photocathode film is positioned on the front side of the substrate. The aforementioned substrate is incorporated into the electron-optical column, and, The aforementioned method, When the photocathode film is placed on the front side of the substrate, it is exposed to the vacuum in the electron-optical column, and The further includes exposing the rear side of the substrate to air. The method according to claim 15.

25. The target is a first target, the irradiation is performed using the laser configured to have a first laser power, and the method is After performing the irradiation, extraction, and direction, the laser is reconfigured to have a second laser power different from the first laser power, and The process further includes inspecting a second target by repeatedly irradiating, drawing, and directing using the laser reconfigured to have the second laser power, The method according to claim 15.

26. The irradiation is performed using the laser configured to have a first laser power, and the method is After performing the irradiation, extraction, and direction, the laser is reconfigured to have a second laser power different from the first laser power, and Using the laser reconfigured to have the second laser power, the process further includes repeatedly irradiating, extracting, and directing, The method according to claim 15.

27. The method according to claim 15, wherein the irradiation is performed using circularly polarized light from the laser.

28. The method according to claim 15, wherein the irradiation is performed using pulsed light from the laser.

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