Electron-beam device and method of operating the same

Abstract

This invention discloses an electron beam device comprising a laser and a photocathode film. The photocathode film has a front side and a back side, and emits a plurality of small electron beams when illuminated from the back side using the laser. The electron beam device also includes electrodes for extracting the plurality of small electron beams from the front side of the photocathode film and for controlling the shape of the plurality of small electron beams.

Description

[Technical Field]

[0001] The present invention relates to electron optical devices, and more specifically to generating a plurality of electron beams in an electron beam apparatus. [Previous Technology]

[0002] Scanning electron microscopes (SEMs) have long been used for inspection applications, such as semiconductor wafer inspection. Traditionally, SEMs have had a single electron beam. However, more recently, SEMs with multiple electron beams (i.e., electron microbeams) have been developed. Multiple electron microbeams can be generated from a single electron source by using an aperture array to separate a global electron beam into electron microbeams. The aperture array may be accompanied by a micro-stigmator array, a micro-deflector array, and a microlens array.

[0003] The throughput of a SEM (or other electron beam apparatus) depends on the number of sub-beams: the more sub-beams, the higher the throughput. However, increasing the number of sub-beams presents significant challenges. For example, as the emission angle of the electron source increases, the angular intensity distribution of the global electron beam and therefore the sub-beams becomes less uniform. Moreover, the increased field of view resulting from the increased number of sub-beams leads to higher field curvature, astigmatism, and distortion. [Summary of the Invention]

[0004] In some embodiments, an 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 small electron beams when illuminated from the back side using the laser. The electron beam device also includes electrodes for extracting the plurality of small electron beams from the front side of the photocathode film and for controlling the shape of the plurality of small electron beams.

[0005] In some embodiments, a method of operating an electron beam device includes illuminating a back side of a photocathode film with a laser to cause the photocathode film to emit a plurality of small electron beams. The method also includes extracting the plurality of small electron beams from a front side of the photocathode film and directing the plurality of small electron beams to a target to detect the target.

Implementation Method

[0018] Various embodiments will now be described in detail with reference to the accompanying drawings, examples of which are illustrated. In the following [Description], numerous specific details are set forth to provide a thorough understanding of one of the various described embodiments. However, those skilled in the art will understand that the various described embodiments can be practiced without such specific details. In other examples, well-known methods, procedures, components, circuits, and networks have not been described in detail to avoid unnecessarily obscuring the nature of the embodiments.

[0019] In electro-optical devices, the lensing effect is provided by electric and / or magnetic fields. Components used to generate these fields may be called lenses, and the fields themselves may also be called lenses. 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 initial component of a second lens.

[0020] FIG1 is a cross-sectional side view along an electron optics column of a portion of an electron beam apparatus 100 according to one of some embodiments. In some embodiments, the electron beam apparatus 100 is used for SEM of a target (e.g., a semiconductor wafer). The electron beam apparatus 100 includes a photocathode film 102 disposed on a substrate 108. The photocathode film 102 is patterned to include an emitting region 104 (e.g., a circular region having a diameter d (which may be in the range of 10 µm to 30 µm)) and a non-emitting region 106. The emitting region 104 is surrounded by the non-emitting region 106 and is thinner than the non-emitting region 106. For example, the emitting region 104 has a substantially uniform (within manufacturing tolerances) first thickness, and the non-emitting region 106 has a substantially uniform (within manufacturing tolerances) second thickness that is thicker than the first thickness.

[0021] The photocathode film 102 has a back side facing one of the substrates 108 and a front side facing away from one of the substrates 108. The photocathode film 102 is disposed on one of the front sides of the substrate 108. Light 110 (e.g., laser light) can be used to illuminate the substrate 108 from the back side of the substrate 108 (i.e., from the side opposite to where the photocathode film 102 is disposed). The light 110 is provided by a light source (e.g., laser 501, FIG. 5). The substrate 108 is transparent to the light 110, such that the light 110 illuminates the substrate 108 and the back side of the photocathode film 102. The light 110 illuminates both the emitting region 104 and the non-emitting region 106. According to the photoelectric effect, this illumination causes the emitting region 104 to emit electrons.

[0022] In some embodiments, the photocathode film 102 is made of gold. For example, the photocathode film 102 is a gold film having a stepped layer. The film thickness in the emitting region 104 can be in the range of 10 nm to 20 nm, and the film thickness in the non-emitting region 106 can be at least five times the thickness of the layer in the emitting region 104. The thickness of the non-emitting region 106 prevents electrons from being emitted from the non-emitting region 106. The gold film is a useful photoemitter due to its stability and well-characterized photoemission properties. The work function of the gold film is close to 4.2 eV, which is well matched with the available laser wavelength. Since the maximum energy spread of photoemitted electrons is the energy difference between the photon energy and the work function, by using a frequency-doubled argon laser to provide light 110 at a wavelength of 257 nm (4.8 eV), the energy spread can be maintained at a half-width of 0.6 eV or less.

[0023] Other examples of the photocathode thin film 102 include, but are not limited to, III-V semiconductors, cesium telluride (CsTe), and chalcogenides. In some embodiments, the photocathode thin film 102 is a material having negative electron affinity. In some embodiments, the substrate 108 is sapphire.

[0024] The electron beam device 100 shown in Figure 1 also includes gun electron optics, which include a Wehnelt electrode (Weh) 112, an extractor electrode (Ext) 114, and an anode 116. The Wehnelt electrode 112 is disposed between the front side of the photocathode film 102 and the extractor electrode 114. The extractor electrode 114 is disposed between the Wehnelt electrode 112 and the anode 116. The extractor electrode 114 extracts a small electron beam 120 from the emitting region 104 of the photocathode film 102 and guides the small electron beam 120 into an acceleration region between the extractor electrode 114 and the anode 116. The small electron beam 120 consists of electrons emitted from the emitting region 104 in response to illumination of the photocathode film 102 by light 110. The Vennet electrode 112 controls the beam shape of the electron microbeam 120 emitted from the emitting region 104 of the photocathode film 102. The anode 116 accelerates the electron microbeam to a desired beam energy (BE). Each of electrodes 112, 114, and 116 includes a hole aligned with the emitting region 104 to allow the electron microbeam 120 to travel through the electrode. The photocathode film 102, Vennet electrode 112, extractor electrode 114, and anode 116 are biased by voltages VRS, VWeh, VExt, and VBE, respectively. The anode may be grounded (i.e., VBE = 0).

[0025] In some embodiments, the light 110 is pulsed (e.g., by means of a pulsed laser 501, FIG. 5), resulting in the pulsation of the electron beam 120. In some embodiments, the light 110 is circularly polarized, resulting in the polarization of the electron beam 120 (i.e., the polarimetric wave motion of the electrons therein).

[0026] A beam-limiting aperture (BLA) 118 is positioned after the anode 116 along the z-axis (and along the path of the electron microbeam 120), such that the anode 116 is positioned between the extractor electrode 114 and the beam-limiting aperture 118. The beam-limiting aperture has an aperture size (e.g., diameter) smaller than the aperture size in the anode 116. The aperture size of the beam-limiting aperture 118 is selected to select a central portion of the electron microbeam 120, thus allowing the central portion to pass through while blocking and thus discarding the non-central portions of the electron microbeam 120. In this way, the beam-limiting aperture 118 controls the geometric aberration blurring of the electron microbeam 120. The beam-limiting aperture 118 can be biased with the same voltage VBE as the anode 116 (e.g., via ground).

[0027] The source of the electron beam 120 (which is actually the emitting region 104 of the photocathode film 102) can be modeled using a virtual source 124 (XO VS) behind the substrate 108. In this modeling, a real beam of electrons from the electron beam 120 generated using the extractor electrode 114 is image-formed by crossing the XO RS. The XO RS is the object of the XO VS, and the XO VS is on the image side of the anode 116 and has the same potential VBE as the anode 116 with respect to the beam energy of the electron beam 120. Therefore, the virtual source trajectory 122 is a straight line, which is equivalent to the tangent of the real source trajectory of the electrons from the electron beam 120 in the exit plane of the beam-limiting aperture 118.

[0028] The magnification M from XO RS to XO VS is given by the following formula: and (1) where V BE is the anode potential and V XORS is the beam potential at XO RS. The position of the virtual source 124 is given by the following formula: (2) where L VS is the distance between the virtual source 124 and the anode 116, L RS is the distance between the real source (i.e., the plane of the emission region 104 in the photocathode film 102) and the anode 116, and z XORS is the position of the real beam cross (XO RS) (i.e., the distance between the plane of the emission region 104 and XO RS).

[0029] The amplification M of Equation (1) and the virtual source position LVS of Equation (2) can be estimated for high beam energies (e.g., 30 keV or higher). In this case, the factor ρ in Equation (1) is much greater than 1x because XORS is near the Vennet electrode 112 and the extractor electrode 114, and the potential VXORS is much lower than VBE before the beam is accelerated. Therefore, Equation (1) and Equation (2) can be approximated as M ≈ 2 / 3 = 0.67 and LVS ≈ 4 * (LRS - zXORS) / 3 ≈ 1.33 * LRS, respectively.

[0030] The portion of the photocathode film 102 shown in Figure 1 contains only a single emitting region 104 to emit a single electron beam 120. Similarly, portions of electrodes 112, 114, and 116 shown in Figure 1 each have only a corresponding single aperture and only show a single beam-limiting aperture 118. However, the photocathode film 102 may contain a plurality of emitting regions 104 separated from each other by non-emitting regions 106. Each of the emitting regions 104 emits a separate electron beam 120 in response to illumination of the back side of the photocathode film 102 by light 110. Due to the thickness of the non-emitting regions 106, no electron beams are emitted from the non-emitting regions 106. The resulting plurality of electron beams 120 are extracted from the emitting regions 104 using an extractor electrode 114, shaped using a Venetian electrode 112, and accelerated using an anode 116. Electrodes 112, 114, and 116 each include a plurality of apertures corresponding to a plurality of emission regions 104 and a plurality of electron microbeams 120. Each aperture is aligned with its respective emission region 104 of its respective electron microbeam 120. The beam-limiting aperture 118 shown in Figure 1 is one of a plurality of beam-limiting apertures 118, used to select the central region of the plurality of electron microbeams 120 and block the non-central region. Each beam-limiting aperture 118 selects the central region of its respective electron microbeam 120 and blocks the non-central region. Each beam-limiting aperture 118 is aligned with its respective emission region 104 and the respective aperture of electrodes 112, 114, and 116.

[0031] In some embodiments, each of the emitting regions 104 in the photocathode film has the same diameter d (e.g., within manufacturing tolerances). The emitting regions 104 may be configured in an array and uniformly spaced (e.g., with rotational symmetry). The apertures of electrodes 112, 114, and 116 may have the same configuration, and the plurality of beam-limiting apertures 118 may also have the same configuration.

[0032] FIG2 is a plan view of a patterned photocathode film 200 used in an electron beam apparatus according to some embodiments. The photocathode film 200 is an example of a photocathode film 102 (FIG. 1). The photocathode film 200 includes a plurality of emitting regions 204 surrounded by a non-emitting region 202. The emitting region 204 is an example of the emitting region 104 and the non-emitting region 202 is an example of the non-emitting region 106 (FIG. 1). The emitting regions 204 are arranged in a hexagonal array in the xy plane perpendicular to the z-axis of FIG. 1. The hexagonal array is desirable because hexagons have a relatively high degree of rotational symmetry. Alternatively, the emitting regions 204 may be arranged in a different pattern (e.g., with a different degree of rotational symmetry). In some embodiments, the emission region 204 is circular in the xy plane, having a diameter d ranging from 10 µm to 30 µm and a pitch (i.e., s) ranging from 75 µm to 150 µm.

[0033] The number of emission regions 204 is determined and equal to the number of electron beams 120 emitted from the photocathode film 200. For the hexagonal pattern of FIG2, the total number of electron beams 120 MEB tot is: (1) where Mx is the number of emission regions 204 along one axis of the hexagon (e.g., the x-axis). For example, in the five rings (i.e., Mx = 11) of the hexagonal distribution of emission regions 204 in FIG2, the total number of electron beams 120 is 91 (i.e., MEB tot = 91). In the ten rings (i.e., Mx = 21) of FIG2, the total number of electron beams 120 is 331 (i.e., MEB tot = 331). The number of rings in the hexagon may be higher or lower than the number of rings shown in FIG2.

[0034] The photocathode film 200 can be fabricated by depositing a thick film (e.g., a gold film) onto a substrate (e.g., a sapphire substrate), photolithographically defining and etching holes (e.g., down to the substrate) with a diameter of d and a spacing of s in the film, and subsequently depositing a thin film (e.g., having the same material as the thick film, e.g., gold) on the thick film and in the holes. The photocathode film 200 can be illuminated (e.g., from the back side, using light 110, FIG. 1), causing electrons to be emitted from the emitting region 204 but not from the thicker non-emitting region 202.

[0035] The Venetian electrode 112, extractor electrode 114, anode 116, and a plurality of beam-limiting apertures 118 can be configured in the same way as the emitting region 204 in the photocathode film 200 by an array of apertures (i.e., apertures). The apertures have the same spacing s as the emitting region 204 and are aligned with the emitting region 204. The apertures of the Venetian electrode 112, extractor electrode 114, and anode 116 have diameters larger than the diameter of the emitting region 204. The apertures (i.e., apertures) of the plurality of beam-limiting apertures have diameters smaller than the diameters of the apertures of the Venetian electrode 112, extractor electrode 114, and anode 116 (i.e., the aperture size of the plurality of beam-limiting apertures is smaller than the aperture size of the Venetian electrode 112, extractor electrode 114, and anode 116). Therefore, instead of showing a photocathode film 200, FIG2 can be regarded as showing a conductive (e.g., metal) plate used as one of the Vennet electrode 112, extractor electrode 114, anode 116 or a plurality of confining apertures 118.

[0036] FIG3 is a cross-sectional side view along an electron optics column of a portion of an electron beam apparatus 300 according to one of some embodiments. The electron beam apparatus 300 includes a photocathode film 302, a Venetian electrode 312, and an extractor electrode 314, which may be examples of photocathode film 102, Venetian electrode 112, and extractor electrode 114 (FIG. 1). Alternatively, the photocathode film 302 may be an example of a photocathode film 906 (FIG. 9) (e.g., Venetian electrode 312 and extractor electrode 314 are examples of Venetian electrode 112 and extractor electrode 114). The electron beam apparatus 300 may also include an anode (e.g., anode 116, FIG. 1) and a plurality of beam-limiting apertures (e.g., including beam-limiting aperture 118, FIG. 1) located on the right side of a portion of the electron beam apparatus 300 shown in FIG. 3. The photocathode film 302 can be an example of the photocathode film 200 (Figure 2), wherein the Vennet electrode 312, the extractor electrode 314, the anode, and a plurality of beam-limiting apertures are configured accordingly.

[0037] Figure 3 shows a computer-simulated ray tracing of electron trajectories in electron microbeams 320 (which include a central electron microbeam 320-1) emitted from respective emitting regions 304 of the photocathode film 302. The emitting region 304 (which includes a central emitting region 304-1) may be an example of emitting region 104 (Figure 1) or an area illuminated by an optical microbeam 904 (Figure 9). The trajectory is generated by applying bias voltages VRS, VWH, VExt, and VBE to the photocathode film 302, the Vennet electrode 312, the extractor electrode 314, and the anode. In some embodiments, the emitting region 304 has a diameter in the range of 10 µm to 30 µm, and the apertures of the Vennet electrode 312, the extractor electrode 314, and the anode have a diameter in the range of 50 µm to 150 µm.

[0038] In some embodiments, the Venetian electrode 312, extractor electrode 314, and / or anode have external apertures that are not used for any of the electron microbeams 320 and do not correspond to any of the emission regions 304. These external apertures include an external aperture 324 for the Venetian electrode 312, an external aperture 326 for the extractor electrode 314, and an external aperture of the anode that can be aligned with the external apertures 324 and 326. These external apertures are dummy apertures (i.e., dummy holes) that enhance the uniformity of the electric field around the aperture through which the electron microbeams 320 travel. For example, a dummy aperture surrounds the aperture through which the electron microbeams 320 travel. In an example of FIG. 2 in which a plate serving as one of the Venetian electrode 112, extractor electrode 114, or anode 116 is shown, one or more of the outer rings of the apertures in the hexagon may be dummy apertures. Dummy apertures reduce the difference in electron trajectories between the internal and external electron microbeams 320 (e.g., ensuring that the difference is negligible).

[0039] Figure 4 is an enlarged view of a central electron beam 320-1 in an electron beam device 300 (Figure 3) according to some embodiments. The central electron beam 320-1 is emitted from a central emission region 304-1 of a photocathode film 302. The electron trajectory and optical properties of the central electron beam 320-1 can be characterized by the central electron beam 402 and the edge electron beam 404, wherein a true source cross XOR and a true image plane 406 are formed and displayed.

[0040] Returning to Figure 2, in one example, the photocathode film 200 has 331 hexagonally distributed emitting regions 204 (i.e., forming 10 rings, where Mx = 21). The emitting regions 204 may have a diameter of 20 µm and a spacing of 100 µm. All emitting regions 204 may be illuminated by a laser beam with a diameter Di = 2000 µm. Based on experimental quantum efficiency measurements of a gold film with a thickness of 10 nm to 20 nm, a 2 W, 257 nm laser can provide a small electron beam current of 1 nA from each of the 331 emitting regions 204 for a total current of 331 nA from all 331 emitting regions 204. The laser power is adjustable from 1 W to 10 W, therefore the current of each small electron beam is adjustable from 0.5 nA to 5 nA for a total current of 165 nA to 1650 nA from all 331 emitting regions 204. These small electron beams and corresponding beam currents can provide high-throughput target (e.g., wafer) inspection and viewing in a single SEM.

[0041] Other examples of adjusting the laser power to the current of a plurality of electron beams are possible. The current of a plurality of electron beams can be adjusted by adjusting the laser power while keeping the settings of the electron optics in the vacuum chamber 130 unchanged, without having to recalibrate the electron optics column.

[0042] Optical lenses can be used to amplify the beam size of a laser beam to illuminate an array of emitting regions 104 on a photocathode film 102 (FIG. 1). FIG. 5 shows an optical device 500 according to some embodiments, which includes a pair of lenses 504 and 506 arranged in series along the z-axis to amplify the beam size of a laser beam 502. A laser 501 (e.g., having adjustable laser power) generates a laser beam 502, which is an example of light 110 (FIG. 1). Therefore, the pair of lenses 504 and 506 may be part of an electron beam device 100 (FIG. 1) (e.g., electron beam device 300, FIG. 3-4). When output from a laser, the laser beam 502 has a beam size Do. After passing through the first lens 504 and the second lens 506, the laser beam 502 has a magnified beam size Di (e.g., Di greater than or equal to 2000 µm) sufficient to illuminate the emitting regions 104 on the photocathode film 102 (e.g., all emitting regions 204 on the photocathode film 200, FIG. 2). The first lens 504 focuses the laser beam 502 onto a cross XO, and the second lens 506 collimates the laser beam 502. The resulting optical magnification is Di / Do = f2 / f1, where f1 is the focal length of the first lens 504 and f2 is the focal length of the second lens 506. In some embodiments, the optical magnification is in the range of 5x to 10x (i.e., a factor of 5 to 10). An amplified laser beam 502 with a magnified beam size Di passes through the back side of the substrate 108 to illuminate the photocathode film 102, thereby inducing photoemission in the emission region 104 of the photocathode film 102, resulting in the emission of a small electron beam 120 from the emission region 104. The optical amplification of the laser beam 502 does not change the power or wavelength of the laser beam 502.

[0043] In some embodiments, the electron beam device 100 (FIG. 1) (e.g., electron beam device 300, FIG. 3-4) includes an image lens to focus a plurality of electron microbeams onto an intermediate image plane. FIG. 6 is a cross-sectional side view of an electron beam device 600 (e.g., electron beam device 100, FIG. 1; electron beam device 300, FIG. 3-4) along a cross-section of an electron optics column, wherein the image lens includes an image lens array (ILA) 602 disposed between an anode 116 and an intermediate image plane 604 (and thus between a plurality of beam-limiting apertures 118 and the intermediate image plane 604) to focus a plurality of electron microbeams 606 onto the intermediate image plane 604. The image lens array 602 includes a respective lens that focuses each electron microbeam 606 onto the intermediate image plane 604. For example, the image lens array 602 includes an image lens for each of the plurality of electron microbeams 606 to focus the electron microbeams 606 onto the intermediate image plane 604.

[0044] The lens of the image lens array 602 can be an electrostatic lens or a magnetic lens. For example, the lens can be a single lens (Einzel lens), such as an electrostatic lens having three electrode plates: a left plate 624 closest to the anode 116, a right plate 628 closest to the central image plane 604, and an 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 implemented according to FIG2, having hexagonally distributed apertures. The aperture size can be equal to or approximately equal to the aperture size in the anode 116. The left plate 624 and the right plate 628 can be grounded, and the intermediate plate 626 can be biased with a voltage VIL for focusing the electron beam 606.

[0045] FIG7 is a cross-sectional side view of an electron beam apparatus 700 (e.g., electron beam apparatus 100, FIG1; electron beam apparatus 300, FIG3-4) along an electron optical column, wherein the image lens includes a global imaging lens (IL) 702 disposed between an anode 116 and an intermediate image plane 704 (and therefore between a plurality of beam-limiting apertures 118 and the intermediate image plane 704) to focus a plurality of electron microbeams 706 onto the intermediate image plane 704, thereby imaged in the intermediate image plane 704. The global imaging lens 702 having a single aperture for one of the plurality of electron microbeams 706 forms a cross 708 for one of the plurality of electron microbeams 706. The cross 708 is located between the global imaging lens 702 and the intermediate image plane 704. The global imaging lens 702 may be an electrostatic lens or a magnetic lens. A magnetic global imaging lens allows for a high reduction ratio in the upper column because the image distance can be shortened without introducing high voltage problems.

[0046] The electron beam apparatus 700 also includes a field lens (FL) 710 to collimate a plurality of small electron beams 706 after the crossing 708. The field lens 710 is a global lens having a single aperture for one of the plurality of small electron beams 706. The field lens 710 coincides with an intermediate image plane 704. For example, the intermediate image plane 704 is in the principal plane of the field lens 710. The field lens 710 may be an electrostatic lens or a magnetic lens.

[0047] The electron beam apparatus 600 (FIG. 6) and electron beam apparatus 700 (FIG. 7) include an upper pillar and a lower pillar. The upper pillar can be considered as extending from the virtual source plane 608 where the virtual source 124 is located (or alternatively, from the substrate 108) to the intermediate image plane 704. The lower pillar can be considered as extending from the intermediate image plane 704 to a target 622 to be detected (e.g., a semiconductor wafer) (or alternatively, to a stage on which the target 622 is mounted).

[0048] The lower pillar of the electron beam apparatus 600 and 700 includes a transfer lens (TL) 610 and an objective lens (OL) 620. The transfer lens 610 is disposed on the side of the intermediate image plane 604 or 704 opposite to the image lens (e.g., the image lens array 602 (FIG. 6), or the global image lens 702 (FIG. 7)). The transfer lens 610 forms one intersection 618 of a plurality of electron microbeams 606 or 706. The intersection 618 is located between the transfer lens 610 and the objective lens 620. In some embodiments, the intersection 618 is closer to the objective lens 620 than to the transfer lens 610, resulting in a reduction in the size of the plurality of electron microbeams 606 or 706. The objective lens 620 focuses the plurality of electron microbeams 606 or 706 onto the target 622 and also decelerates the plurality of electron microbeams 606 or 706 to a landing energy. Objective lens 620 is positioned between crossbeam 618 and target 622 (and therefore between the stage of crossbeam 618 and target 622). Transfer lens 610 is positioned between intermediate image plane 604 or 704 and crossbeam 618. In electron beam apparatus 700, transfer lens 610 is positioned between field lens 710 and objective lens 620. Transfer lens 610 may be an electrostatic lens or a magnetic lens. Objective lens 620 may be an electrostatic lens or a hybrid magnetic-electrostatic lens. Transfer lens 610 and objective lens 620 are global lenses having a single aperture for each of the plurality of electron microbeams 606 or 706 (i.e., through which the plurality of electron microbeams 606 or 706 travel).

[0049] The plurality of electron beams 606 or 706 incident on the transfer lens 610 are collimated so that the electron beams 606 or 706 telecentrically illuminate the transfer lens 610. In the example of the electron beam device 700 (FIG. 7), the field lens 710 collimates the plurality of electron beams 706 so that the plurality of electron beams 706 telecentrically illuminate the transfer lens 610.

[0050] The lower column electron optics of electron beam devices 600 and 700 are projection optics. The projection optics can have a large optical reduction ratio (e.g., ~10x), with the cross 618 located near the objective lens 620 to reduce both the effect of Coulomb interactions on resolution and the off-axis aberration of the outer ring electron beams 606 or 706. Optical performance (i.e., the resolution of individual electron beams 606 or 706) is primarily dominated by the Coulomb interactions around the cross 618. Using a purely electrostatic objective lens 620, the effect of Coulomb interactions is greatly reduced, thus improving resolution. For relatively low beam currents, the effect of Coulomb interactions around the cross 618 is reduced, but the source image resolution can be degraded due to the larger virtual source size of the plurality of emission regions 104.

[0051] According to computer simulations, for current rise measurements of 20% to 80%, the actual source cross (XO RS) size can be approximately 300 nm. The virtual source 124 (XO VS) size can be approximately 0.67 * 300 nm ≈ 200 nm. Using a purely electrostatic objective lens 620, the optical reduction ratio in the column below the electron beam device 600 or 700 can be approximately 10x. The optical reduction ratio in the column above the electron beam device 700 (Figure 7) can be higher than 5x, and for relatively low beam currents (e.g., single-electron small beam currents less than 1 nA), a source image better than 200 / (10 * 5) = 4 nm is given at the target 622. For higher beam currents, the blurring due to Coulomb interactions around the cross 618 can dominate the source image. The Coulomb interaction between the electron microbeams 606 or 706 listed above is negligible because the separation between electron microbeams (e.g., greater than or equal to 100 µm) is much greater than the average separation of electrons in a single electron microbeam 606 or 706.

[0052] 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 excessive electrical intensity across one of the gaps in the image lens array 602 (FIG. 6) or the global image lens 702 (FIG. 7). For example, the image distance can be greater than or equal to 50 mm, wherein the length of the column from the virtual source plane 608 to the intermediate image plane 604 or 704 is at least 300 mm.

[0053] In addition to the transfer lens 610 and the objective lens 620, the lower column of the electron beam devices 600 and 700 also includes 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. Therefore, 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.

[0054] A second Wayne filter 616 deflects secondary electrons 630 from the target 622 to a detector 614 (e.g., a detector array), which detects the secondary electrons 630. The secondary electrons 630 are electrons from a plurality of small electron beams 606 or 706 scattered from the target 622. By deflecting the secondary electrons 630 from the target 622 to the detector 614, the second Wayne filter 616 collects the secondary electrons 630, thus allowing detection of the target 622.

[0055] The second Wayne filter 616 is strong enough to allow secondary electrons 630 with a deflection angle wide enough for proper imaging of the target 622 to be directed to the detector 614. This strong Wayne filter generates energy dispersion due to the presence of source energy dispersion (e.g., 0.6 eV or less, as previously stated). This energy dispersion causes blurring of the plurality of electron microbeams 606 or 706. The first Wayne filter 612 compensates for this energy dispersion by correcting the second Wayne filter 616: the energy dispersion generated by the first Wayne filter 612 compensates for the energy dispersion generated in the second Wayne filter 616, thus reducing or eliminating blurring.

[0056] FIG8 is a cross-sectional view of a magnetic lens stack 800 having a shared magnetic pole piece 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 a global image lens 702 (FIG. 7). The field lens 808 and the transfer lens 810 are respective examples of the field lens 710 and the transfer lens 610 (FIG. 7). The magnetic lens stack 800 having a shared magnetic pole piece 802 allows for strict 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 also be used to correct the image rotation of a plurality of small electron beams 706 by applying opposite currents (i.e., currents in opposite directions) to different coils of the respective lenses 804, 806, 808, and 810.

[0057] FIG9 illustrates an optical element 900 of an electron beam apparatus according to one of some embodiments, wherein a patterned photocathode film 102 is replaced by a photocathode film 906, which emits a plurality of electron beams when illuminated from the back by a plurality of optical microbeams 904. The photocathode film 906 is disposed on a substrate 108. The photocathode film 906 may be unpatterned (e.g., flat) and sufficiently thin (e.g., having a thickness of 10 nm to 20 nm) to allow photoemission in response to illumination by the plurality of optical microbeams 904. The plurality of optical microbeams 904 illuminate respective regions 908 of the photocathode film 906. When each region 908 is illuminated by the plurality of optical microbeams 904, a plurality of electron beams (e.g., electron beam 320, FIG3) are emitted from each region 908. No electron beams are emitted from unilluminated regions of the photocathode film 906. 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.

[0058] Optical device 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 amplify the laser beam 502. An aperture array 902 divides the laser beam 502 (e.g., amplified by the pair of lenses 504 and 506) into a plurality of smaller optical beams 904. The aperture array (LAA) 902 is disposed between the laser 501 and the back surface of the photocathode film 906 (and between the laser 501 and the substrate 108). (The back surface of the photocathode film 906 is the side facing the substrate 108.) For example, the pair of lenses 504 and 506 are disposed between the laser 501 and the aperture array 902, such that the aperture array 902 is disposed between the second lens 506 and the substrate 108. In some embodiments, the aperture array 902 is deposited on the substrate 108. Because the plurality of small optical beams 904 of the illuminating photocathode film 906 originate from laser 501, the photocathode film 906 is illuminated by laser 501. The aperture array 902 is an aperture array; in some embodiments, the apertures of the aperture array 902 are arranged in a hexagonal pattern, as shown in FIG2. In some embodiments, the aperture size (e.g., diameter) of the aperture array 902 is in the range of 10 µm to 30 µm.

[0059] In some embodiments, the laser 501 is pulsed, resulting in pulsation of the laser beam 502, the plurality of optical microbeams 904, and the plurality of electron microbeams emitted from the photocathode film 906. In some embodiments, the laser beam 502 is circularly polarized, resulting in the circular polarization of the plurality of optical microbeams 904 and the corresponding polarization of the plurality of electron microbeams emitted from region 908 of the photocathode film 906.

[0060] The optical device 900 and the photocathode film 906 can be combined with the electron optical column of the electron beam device 100 (FIG. 1) (e.g., the electron beam device 300 (FIG. 3 to 4)). For example, the optical device 900 and the photocathode film 906 can be combined with the electron optical column of the electron beam device 600 (FIG. 6) or 700 (FIG. 7).

[0061] As discussed, the photocathode film 102 or 906 is disposed on the front side of a substrate 108. In some embodiments, the substrate 108 is integrated into a vacuum chamber 130 of an electron optics column. The front side of the substrate 108 faces the vacuum chamber 130, such that the photocathode film 102 or 906 is located within the vacuum chamber 130. The back side of the substrate 108, to be illuminated by a laser (e.g., laser 501, FIG. 5 or FIG. 9), is exposed to air. Thus, the optics of the electron beam device can be in air, while the electron optics are in a vacuum.

[0062] Figure 10 is a flowchart illustrating a method 1000 of operating an electron beam apparatus (e.g., electron beam apparatus 100, Figure 1; 300, Figures 3-4; 600, Figure 6; 700, Figure 7) according to some embodiments. In method 1000, a back side of a photocathode film is illuminated (1002) using a light source, which may be a laser (e.g., laser 501, Figure 5 or Figure 9), causing the photocathode film to emit a plurality of small electron beams. The light illuminating the photocathode film may be pulsed and / or circularly polarized, resulting in pulsation and / or polarization of the plurality of small electron beams.

[0063] In some embodiments, the photocathode film (e.g., photocathode film 102, FIG. 1; 200, FIG. 2) comprises (1004) a plurality of emitting regions (e.g., emitting region 104, FIG. 1; 204, FIG. 2), which are separated from each other by a non-emitting region (e.g., non-emitting region 106, FIG. 1; 202, FIG. 2). The plurality of emitting regions are thinner than the non-emitting region. Illuminating the back side of a photocathode film causes the photocathode film to emit a plurality of small electron beams from the emitting regions.

[0064] In some other embodiments, a laser beam (e.g., laser beam 502, FIG. 9) is generated (1006) using a laser, and the laser beam is divided into a plurality of smaller optical beams (e.g., smaller optical beams 904, FIG. 9) using an aperture array (e.g., aperture array 902, FIG. 9). The plurality of smaller optical beams illuminate (1006) respective regions (e.g., region 908, FIG. 9) of the photocathode film (e.g., photocathode film 906, FIG. 9) from the back side of the photocathode film.

[0065] A plurality of electron beams are extracted (1008) from the front side of one of the photocathode films. An extractor electrode (e.g., extractor electrode 114, FIG. 1, 6 and / or FIG. 7; 314, FIG. 3 to 4) can be used to extract the plurality of electron beams.

[0066] In some embodiments, a plurality of electron microbeams are extracted (1010) from the emission region (e.g., emission region 104, FIG. 1; 204, FIG. 2) in step 1004. In some other embodiments, in step 1006, a plurality of electron microbeams are extracted (1012) from each region (e.g., region 908, FIG. 9) illuminated by a plurality of optical microbeams.

[0067] A plurality of beam-limiting apertures (e.g., a plurality of beam-limiting apertures 118, FIG1, FIG6 and / or FIG7) can be used to select the central portion of a plurality of electron microbeams and can block (1014) the non-central portion of a plurality of electron microbeams.

[0068] A plurality of electron microbeams (1016) are guided to a target (e.g., target 622, FIG. 6 or FIG. 7) for target detection. In some embodiments, the target is a semiconductor wafer. Guiding the plurality of electron microbeams may include: controlling the shape of the plurality of electron microbeams using a Vennet electrode (e.g., Vennet electrode 112, FIG. 1, FIG. 6 and / or FIG. 7; 312, FIG. 3 to 4) and accelerating the plurality of electron microbeams using an anode (e.g., anode 116, FIG. 1, FIG. 6 and / or FIG. 7). The Vennet electrode is disposed between the front side of the photocathode film and the extractor electrode. The extractor electrode is disposed between the Vennet electrode and the anode. The anode may be disposed between the extractor electrode and a plurality of beam-limiting apertures and may have openings for the plurality of electron microbeams, the openings having an aperture size larger than one aperture size of the plurality of beam-limiting apertures.

[0069] In some embodiments, as part of guiding a plurality of electron microbeams to a target, the plurality of electron microbeams are focused (1018) onto an intermediate image plane (e.g., intermediate image plane 604 or 704, FIG. 6 or FIG. 7). In some embodiments, this focusing is performed using an image lens array (e.g., image lens array 602, 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, FIG. 7) disposed between the anode and the intermediate image plane. Focusing performed using a global image lens includes forming one of the electron microbeam intersections (e.g., intersection 708, FIG. 7) between the global image lens and the intermediate image plane, and guiding the plurality of electron microbeams to a target further includes collimating the plurality of electron microbeams focused onto the intermediate image plane. Collimation can be performed using a field lens (e.g., field lens 710, FIG. 7).

[0070] A transfer lens (e.g., transfer lens 610, FIG. 6 or 7) is used to form (1020) an intersection (e.g., intersection 618, FIG. 6 or 7) of one of the plurality of electron microbeams located on the side opposite the anode in the intermediate image plane. (This intersection is different from an intersection that can be formed by a global image lens.) An objective lens (e.g., objective lens 620, FIG. 6 or 7) is used to focus and decelerate (1022) the plurality of electron microbeams onto the target. The objective lens is positioned between this intersection and the target.

[0071] Method 1000 may further include the use of a first and a second Wayne filter. A second Wayne filter (e.g., second Wayne filter 616, Figure 6 or Figure 7), positioned closer to the objective lens than the first Wayne filter (e.g., first Wayne filter 612, Figure 6 or Figure 7), deflects secondary electrons from the target to a detector (e.g., detector 614, Figure 6 or Figure 7). The first Wayne filter compensates for the effect of the second Wayne filter on the plurality of electron beams.

[0072] In some embodiments of method 1000, a photocathode film is disposed on a front side of a substrate (e.g., substrate 108, FIG. 1, 6, 7 and / or FIG. 9) integrated into an electron optical column. The photocathode film disposed on the front side of the substrate is exposed to a vacuum within the electron optical column (e.g., in a vacuum chamber 130, FIG. 1, 3 to 7 and / or FIG. 9). The back side of the substrate is exposed to air (e.g., air 132, FIG. 1, 3 to 7 and / or FIG. 9).

[0073] The method 1000 can be repeated multiple times. For example, different repetitions of the method 1000 can be performed to detect different targets and / or repeatedly detect the same target. The laser power can be adjusted between repetitions. Thus, different laser powers and therefore different electron beam currents corresponding to different laser powers can be used to detect different targets and / or repeatedly detect the same target. For example, a first repetition of the method 1000 can be performed for a first target, wherein the laser is configured to have a first laser power. After the first repetition is completed, the laser can then be reconfigured to have a second laser power that is different from (e.g., greater than or less than) the first laser power. Then, a second repetition of the method 1000 can be performed for a second target using the laser as reconfigured to have the second laser power. Alternatively, a second repetition can be performed for a first target using a laser as reconfigured to have the second laser power. In some embodiments, the setting of the electron optical components in the vacuum of the electron optical column remains unchanged between repetitions, and the electron optical column is not recalibrated between repetitions. Instead, the laser was reconfigured to change the laser power without adjusting the settings of the electron optics in the vacuum or recalibrating the electron optics column.

[0074] Although the operations of method 1000 appear to occur in a specific order in Figure 10, they can all be executed simultaneously (e.g., on an ongoing basis). Method 1000 may contain more or fewer operations than shown. Two or more operations may be combined into a single operation.

[0075] For illustrative purposes, the foregoing description has been described with reference to specific embodiments. However, the illustrative discussion above is not intended to be exhaustive or to limit the scope of the invention's claims to the precise forms disclosed. In view of the foregoing teachings, many modifications and variations are possible. The embodiments have been selected to best illustrate the basic principles of the invention's claims and their practical application, thereby enabling others skilled in the art to best use the embodiments in combination with various modifications suitable for the particular purpose considered. [Simplified Explanation of the Diagram]

[0006] To better understand the various described embodiments, reference should be made to the following [Implementation] in conjunction with the following figures.

[0007] Figure 1 is a cross-sectional side view of a portion of an electron beam apparatus along an electron optical column according to one of some embodiments.

[0008] Figure 2 is a plan view of a patterned photocathode film used in an electron beam apparatus according to some embodiments.

[0009] Figure 3 is a cross-sectional side view of a portion of an electron beam apparatus along an electron optical column according to one of some embodiments, and shows a computer-simulated electron trajectory of a plurality of electron microbeams.

[0010] Figure 4 is an enlarged view of one of the central electron beams in the electron beam apparatus of Figure 3 according to some embodiments.

[0011] Figure 5 shows a pair of lenses according to some embodiments for enlarging the beam size of a laser beam used to illuminate a photocathode film to induce photoelectric emission.

[0012] FIG6 is a cross-sectional side view of an electron beam apparatus according to one of some embodiments along an electron optical column, the electron beam apparatus including an image lens array disposed between an anode and an intermediate image plane to focus a plurality of electron microbeams onto the intermediate image plane.

[0013] FIG7 is a cross-sectional side view of an electron beam apparatus according to one of some embodiments along an electron optical column, the electron beam apparatus including a global imaging lens disposed between an anode and an intermediate image plane to focus a plurality of electron microbeams onto an intermediate image plane.

[0014] FIG8 is a cross-sectional view of a magnetic lens stack having a common magnetic pole piece according to some embodiments.

[0015] Figure 9 shows an optical device of an electron beam apparatus according to some embodiments, which has a photocathode film that emits a plurality of electron beams when illuminated from the back by a plurality of optical beams.

[0016] Figure 10 is a flowchart illustrating one method of operating an electron beam device according to some embodiments.

[0017] The same component symbols are used throughout the diagrams and instructions to refer to the corresponding parts.

Claims

1. An electron beam device, comprising: One laser; A photocathode film having a front side and a back side, for emitting a plurality of small electron beams when illuminated from the back side by a laser; The device includes electrodes for extracting the plurality of electron beams from the front side of the photocathode film and for controlling the shape of the plurality of electron beams. The electrodes include: a Wehnelt electrode having a first plurality of openings for the plurality of electron beams; an extractor electrode having a second plurality of openings for the plurality of electron beams; and an anode having a third plurality of openings for the plurality of electron beams, wherein: the Wehnelt electrode is disposed between the front side of the photocathode film and the extractor electrode; and the extractor electrode is disposed between the Wehnelt electrode and the anode.

2. The electron beam apparatus as claimed in claim 1, wherein: The photocathode film includes a plurality of emitting regions separated from each other by a non-emitting region; the plurality of emitting regions are thinner than the non-emitting region; and when the back side of the photocathode film is illuminated by the laser, the plurality of small electron beams are emitted from the plurality of emitting regions.

3. The electron beam apparatus as described in claim 2, wherein: The photocathode thin film is made of gold; and the plurality of emitting regions have a first thickness of 10 nm to 20 nm.

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

5. The electron beam apparatus of claim 1, further comprising an aperture array disposed between the laser and the back surface of the photocathode film to divide a laser beam from the laser into a plurality of small optical beams to illuminate respective regions of the photocathode film from the back surface of the photocathode film; wherein when the respective regions are illuminated by the plurality of small optical beams, the plurality of small electron beams are emitted from the respective regions.

6. The electron beam apparatus of claim 5, further comprising a pair of lenses disposed between the laser and the aperture array to amplify the laser beam.

7. The electron beam apparatus of claim 1, further comprising a plurality of beam-limiting apertures to select the central portion of the plurality of electron beams and block the non-central portions of the plurality of electron beams, wherein: The anode is positioned between the extractor electrode and the plurality of confining apertures; and one of the third plurality of openings of the anode has an aperture size larger than one of the plurality of confining apertures.

8. The electron beam apparatus of claim 1, further comprising an image lens for focusing the plurality of electron beams onto an intermediate image plane.

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

10. The electron beam apparatus as claimed in claim 8, wherein: The imaging lens includes a global imaging lens positioned between the anode and the intermediate image plane to form one of the plurality of electron beams crossing, and after the crossing, the plurality of electron beams are focused onto the intermediate image plane, wherein the crossing is located between the global imaging lens and the intermediate image plane; and the electron beam device further includes a field lens that coincides with the intermediate image plane to collimate the plurality of electron beams.

11. The electron beam apparatus of claim 8, further comprising: A transfer lens is positioned on the side of the intermediate image plane opposite to the image lens to form an intersection of the plurality of electron microbeams; and an objective lens is used to focus the plurality of electron microbeams onto a target; wherein the intersection is located between the transfer lens and the objective lens.

12. The electron beam apparatus of claim 11, further comprising a first Wayne filter and a second Wayne filter disposed between the transfer lens and the objective lens, wherein: The second Wayne filter is closer to the objective lens than the first Wayne filter; the second Wayne filter will deflect secondary electrons from the target to a detector; and the energy dispersion of one of the first Wayne filters compensates for the energy dispersion of one of the second Wayne filters.

13. The electron beam apparatus of claim 1, wherein: The photocathode film is disposed on one of the front sides of a substrate; the substrate is integrated into an electron optical column, the electron optical column including a vacuum chamber; the front side of the substrate faces the vacuum chamber, the photocathode film is located inside the vacuum chamber; and one of the back sides of the substrate to be illuminated by the laser is exposed to air.

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

15. A method of operating an electron beam device, comprising: A laser is used to illuminate the back side of a photocathode film, causing the photocathode film to emit a plurality of small electron beams; An extractor electrode is used to extract the plurality of electron beams from one of the front sides of the photocathode film; and the plurality of electron beams are guided to a target for detection, the guidance comprising: using a Vennet electrode to control the shape of the plurality of electron beams, and using an anode to accelerate the plurality of electron beams, wherein: the Vennet electrode is disposed between the front side of the photocathode film and the extractor electrode; and the extractor electrode is disposed between the Vennet electrode and the anode.

16. As in request item 15, wherein: The photocathode film includes a plurality of emitting regions separated from each other by a non-emitting region; the plurality of emitting regions are thinner than the non-emitting region; the illumination causes the photocathode film to emit the plurality of electron beams from the emitting regions; and the extraction includes extracting the plurality of electron beams from the emitting regions.

17. As in request item 15, wherein: The illumination includes: generating a laser beam using the laser, dividing the laser beam into a plurality of small optical beams using an aperture array, and illuminating each region of the photocathode film from the back side of the photocathode film using the plurality of small optical beams; and the extraction includes extracting the plurality of small electron beams from the respective regions.

18. The method of claim 15, further comprising using a plurality of beam-limiting apertures to select the central portion of the plurality of electron microbeams and to block the non-central portion of the plurality of electron microbeams, wherein: The anode is positioned between the extractor electrode and the plurality of beam-limiting apertures; and one of the apertures in the anode for the plurality of electron beams has a size larger than one of the apertures in the plurality of beam-limiting apertures.

19. The method of claim 15, wherein the guidance further includes focusing the plurality of electron microbeams onto an intermediate image plane.

20. The method of claim 19, wherein an image lens array disposed between the anode and the intermediate image plane is used to perform focusing the plurality of electron beams onto the intermediate image plane.

21. As in request item 19, wherein: Using a global imaging lens disposed between the anode and the intermediate image plane to perform focusing the plurality of electron beams onto the intermediate image plane; focusing the plurality of electron beams onto the intermediate image plane includes: forming one intersection of the plurality of electron beams between the global imaging lens and the intermediate image plane; and the guidance further includes using a field lens to collimate and focus the plurality of electron beams onto the intermediate image plane.

22. The method of request item 19, wherein the guidance further includes: A transfer lens is used to form one of the plurality of electron beams crossing, wherein the crossing is located on the side of the intermediate image plane opposite to the anode; and an objective lens is used to focus and decelerate the plurality of electron beams onto the target, wherein the objective lens is positioned between the crossing and the target.

23. The method of claim 22, further comprising: A first Wayne filter is used to compensate for the effect of a second Wayne filter on the plurality of electron beams, wherein the second Wayne filter is closer to the objective lens than the first Wayne filter; and the second Wayne filter is used to deflect secondary electrons from the target to a detector.

24. As in request item 15, wherein: The photocathode film is disposed on one of the front sides of a substrate; The substrate is integrated into an electron optical column; The method further includes: exposing the photocathode film, such as that disposed on the front side of the substrate, to a vacuum within the electron optical column; And expose the back side of the substrate to air.

25. The method of claim 15, wherein the target is a first target and the illumination is performed using a laser configured to have a first laser power, the method further comprising: After performing the illumination, extraction, and guidance, the laser is reconfigured to have a second laser power different from one of the first laser powers; And using the laser, which has been reconfigured to have the second laser power, to repeat the illumination, the extraction, and the guidance to detect a second target.

26. The method of claim 15, wherein the illumination is performed using a laser configured to have a first laser power, the method further comprising: After performing the illumination, extraction, and guidance, the laser is reconfigured to have a second laser power different from one of the first laser powers; And the illumination, extraction and guidance are repeated using the laser that has been reconfigured to have the second laser power.

27. The method of claim 15, wherein the illumination is performed using circularly polarized light from the laser.

28. The method of claim 15, wherein the illumination is performed using pulsed light from the laser.

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