scanning electron microscope

Abstract

Scanning electron microscope comprising an electron source (1) and a monochromator (20) which makes an electron beam (3) emitted by the electron source monochromatic, wherein: the monochromator comprises a magnetic field generator (21) that deflects the electron beam and an energy selection aperture (23) that allows a portion of the electron beam deflected by the magnetic field generator to pass through; the magnetic field generator is arranged on a trajectory of electrons emitted by a sample (11) and is configured to deflect the electrons emitted by the sample onto different trajectories by means of a magnetic field generated by it when the sample is irradiated with the electron beam that has passed through the monochromator; and A first detector (29) and a first aperture (27) arranged between the magnetic field generator and the corresponding first detector are provided on each of the different trajectories.

Description

Technical area

[0001] The present invention relates to a monochromator and a scanning electron microscope capable of monochromatizing an electron beam emitted by an electron source. More specifically, the invention relates to a monochromator and a scanning electron microscope capable of energy discrimination of electrons emitted by a sample. State of the art

[0002] There is a growing trend toward miniaturization of semiconductor devices that can be measured and observed with scanning electron microscopes, necessitating ever-increasing resolutions for scanning electron microscopes. Patent Literature 1 describes a monochromator that enables increased resolution. The monochromator disperses the energy of an electron beam and selectively detects electrons with a specific energy, making it possible to make the beam monochromatic. Patent Literature 2 describes a scanning electron microscope that includes a multiple prism array arranged in the beam path to suppress aberration. The microscope detects secondary electrons separated from the electron beam trajectory by utilizing the deflection effect of the multiple prism array.

[0003] A scanning electron microscope with an electron energy filter acting as a monochromator, which deflects electrons emitted by a sample to a detector, is disclosed in DE 197 46 785 A1. Further electron microscopes with beam splitters arranged on the optical axis of the primary electron beam for deflecting the electrons emitted by the sample are described in the publications DE 102 36 738 A1, US 6 310 341 B1, US 2004 / 0075053 A1, and US 2007 / 0181805 A1. Literature listPatent literature Patent literature 1: JP 2007-184139 A (corresponding to US patent US 7,612,336 B2 Patent literature 2: JP H09 - 270 241 A Summary of the inventionTechnical problem

[0004] In addition to miniaturization, semiconductor devices are being realized with ever-increasing aspect ratios. For example, contact windows are becoming deeper, with aspect ratios of 30 or more being offered. In conjunction with an increasing number of stacked layers, patterns have been developed in which a plurality of materials are coexisting within a contact window. Thus, the electron microscope must have the ability to identify and measure such patterns formed with different materials. This means that the equipment for measuring and inspecting the latest semiconductor devices must have both higher resolution and the ability to identify the pattern composition.While Patent Publications 1 and 2 describe techniques for achieving higher resolution through aberration suppression by isolating a single color, there is no information regarding the identification and measurement of a sample with different compositions.

[0005] A scanning electron microscope is described below that realizes both an increase in resolution and the ability to recognize patterns. Solution to the problem

[0006] To solve this problem, the scanning electron microscope defined in claim 1 is proposed. Further advantageous embodiments are described in the subclaims. Advantageous effects of the invention

[0007] According to the above configuration, both an increase in resolution and the ability of pattern recognition can be achieved. Short description of the drawings Fig. Figure 1 illustrates an example of a scanning electron microscope equipped with a monochromator. Fig. Figure 2 illustrates an example where the trajectory of electrons emitted from a sample is deflected by a magnetic field generator of the monochromator. Fig. Figure 3 explains the relationship between the electron beam aperture angle (focal depth) and the resolution in a case where the monochromator is used and in a case where the monochromator is not used. Description of the embodiments

[0008] For a scanning electron microscope (SEM) to produce a composition image with very high resolution and compositional contrast sensitivity, it is necessary to use an electron beam with low acceleration energy, on the order of a few keV or less, to minimize the range where reflected electrons / backscattered electrons (BSE) containing information about the sample composition are emitted, and to detect the BSE and the like within a fixed angle of about 15° with respect to the electron beam (primary electron beam) incident on the sample. When viewing a sample with a high aspect ratio, some of the electrons emitted from the bottom surface of the sample collide with a side wall of the sample and therefore cannot be detected.Thus, it is necessary to efficiently capture electrons within a fixed angle of a few degrees to the optical axis of the primary electron beam so that the sample can be viewed as a scanning electron microscopy (SEM) image. Furthermore, if a small potential difference exists on the sample, e.g., in the case of a pn junction, the electrons emitted by the sample will have different energies. If the energies can be differentiated, a potential range can be determined.

[0009] A method is provided for increasing the resolution of a low-acceleration SEM by a scanning electron microscope with a monochromator, which solves the problem that a sufficiently small spot diameter cannot be achieved due to blurring caused by chromatic aberration and which selectively allows only a desired energy range to pass with respect to the primary electron beam.

[0010] To enhance the pattern recognition capability described above, an image with high compositional contrast must be generated. The electrons emitted by the sample must have different emission energies depending on the sample's composition. Thus, compositional contrast can be enhanced by selectively detecting electrons with a specific energy through energy discrimination.

[0011] One means of achieving such energy discrimination is an energy filter. The energy filter comprises, for example, a plurality of mesh electrodes overlapping along the optical axis. The energy filter creates a reversed electric field with respect to the electrons emitted by the sample, so that electrons with lower energy are repelled while electrons with higher energy are selectively detected. While the filter can block weak electrons, it cannot selectively detect weak electrons or select electrons within a specific range of the energy spectrum.

[0012] For example, if a pattern comprising three materials A, B, and C is present in a contact window, where the materials emit electrons with an energy relationship such that A>B>C, the energy filter will selectively pass only the higher-energy electrons. Thus, although the energy filter can selectively detect only the electrons emitted by A or only those emitted by A and B, it cannot selectively detect only the electrons emitted by C or only those emitted by B and C. It is desirable that the apparatus for measuring the latest semiconductor devices with increasingly higher aspect ratios and with an increasing number of stacked layers be capable of selectively detecting electrons with relatively weak energies.

[0013] With reference to the embodiment described below, a scanning electron microscope provided with a monochromator and capable of providing a highly accurate compositional contrast image in a relatively simple configuration without mesh electrodes and the like will be explained.

[0014] Furthermore, a scanning electron microscope is described which is capable of measuring a pattern comprising a plurality of materials contained in a sample with a high aspect ratio by enabling both selective detection of electrons emitted at a solid angle of a few degrees from the optical axis of the primary electron beam and discrimination of energy.

[0015] According to the present embodiment, a scanning electron microscope is described in particular, which achieves increased resolution by isolating a single color, improves the contrast of an SEM image of a bottom surface of a high-aspect-ratio sample, and determines a small potential difference by detecting electrons emitted from the sample at a specific solid angle and by an energy discrimination function. Specifically, a scanning electron microscope is described, which is equipped with a monochromator that makes the electron beam monochromatic and has a function for detecting electrons emitted from the sample at a specific solid angle and discriminating energy.

[0016] Furthermore, the present embodiment is configured such that, to reduce the required number of components, a magnetic field generator arranged in the monochromator for deflecting the primary electron beam toward an electric field generator is also used to deflect the electrons emitted from the sample. The electrons deflected in a direction opposite to the primary electron beam are subjected to energy discrimination and detected by a detector, such as a scintillator or photomultiplier, to provide an SEM image.

[0017] Hereinafter, the operation of the scanning electron microscope provided with the monochromator according to the present embodiment will be described with reference to Fig. 1. When an extraction voltage is applied between an electron source 1 and an anode electrode 2, a primary electron beam 3 is emitted from the electron source 1 along a linear optical axis. The primary electron beam 3 comprises a primary electron beam 3a that expands to a few tenths of a mrad. An astigmatism-corrected real image of the electron source 1 is converged by a first convergence lens 4 and a first astigmatism correction lens 5 at an intersection with a symmetry plane 24 of the monochromator.

[0018] Subsequently, the primary electron beam 3 from the electron source 1 is deflected by approximately 160° to an energy selection aperture 23 by a magnetic field generator 21 of a front-stage deflection system. It is then deflected by the same angle in the opposite direction by a first electric field generator 22a. The deflection directs the beam in a direction parallel to the original linear optical axis. The beam is then subjected to energy decomposition in the x-direction at the position of the energy selection aperture 23, which is arranged in an energy decomposition plane, and converged in the y-direction.The beam then converges to a point, while a symmetry plane trajectory is drawn by a second electric field generator 22b of a final-stage deflection system, which is arranged in a symmetrical position with respect to the plane including the energy selection aperture 23 and configured to return the primary electron beam 3 to the linear optical axis of the electron beam, and by the magnetic field generator 21, thereby eliminating the energy decomposition caused by the front-end deflection system and forming an undecomposed virtual image. The virtual image of the electron beam 1, which has been made monochromatic, is formed by a second convergence lens 8 and then reduced by an objective lens 10, forming a fine intersection point on the surface of the sample 11.The aperture angle of the primary electron beam 3 or the current magnitude of the primary electron beam is limited by an aperture 6 installed between the monochromator 20 and the objective lens 10. The primary electron beam 3 is further caused to scan the sample two-dimensionally through two stages of a deflection device 9 using an electrostatic field.

[0019] Electrons 12 emitted from the sample (secondary electrons and / or reflected electrons) travel upward, subjected to the lensing action of the objective lens 10. An accelerating electrode 17, to which a positive electric field can be applied, is installed so that the emitted electrons are further attracted upward toward the detector. Furthermore, a function is provided to apply positive and negative voltages to the sample 11. When a positive voltage is applied, the electrons emitted from the sample can be attracted back toward the sample, so that only the electrons with a certain energy can be guided toward the detector. On the other hand, when a negative voltage is applied, the electrons can be further accelerated and attracted upward toward the detector.The applied voltage for the accelerating electrode 17 and the applied voltage for the sample 11 are controlled by an optical system control device 31 in the electron microscope control device 30 for the respective energy of the electrons as the detection target.

[0020] For sample observation without using the monochromator or for automatic adjustment of the monochromator operating state, a collision with a scintillator 15, to which a high positive voltage is applied, produces a light emission that is converted into an electrical signal and amplified by a photomultiplier 16 to enable observation of a scanning electron microscopic image (SEM image). Fig. 1, the electrons 12a are caused to collide with a conversion electrode 13 during their ascent, thereby generating secondary electrons 14, which are then detected. However, the electrons 12a can be detected directly. The conversion electrode 13 or detector is arranged between the magnetic field generator 21 and the sample 11 and functions as an aperture (second aperture) that limits the passage of electrons within an angular range larger than a predetermined radiation angle with respect to the optical axis of the primary electron beam 3 as the center. For example, when measuring a contact window, among the electrons emitted from the bottom of the window, a larger number of electrons are emitted in a narrow angular range than in a wide angular range.Thus, by limiting the passage of the wide angle emitted electrons, the electrons detected by a detector 29 (first detector) provide improved information about the window bottom.

[0021] When an SEM image is formed using a photomultiplier 29 arranged in the final stage of the monochromator, the electrons 12b emitted from the sample 11 are returned by an auxiliary deflector 19 along the same trajectory as the primary electron beam and then deflected in a direction opposite to the primary electron beam by the magnetic field generator 21 arranged in the monochromator 20.

[0022] The emitted electrons 12b are emitted at a specific solid angle relative to the linear optical axis of the primary electron beam. The convergence of the electrons can be adjusted by the electron convergence lens 18 so that the electrons can pass through the opening of the conversion electrode 13 for the passage of the primary electron beam and the aperture 6.

[0023] The deflected electrons 12b pass through an auxiliary deflection device 25 capable of two-dimensional XY scanning and a limiting aperture 27 (first aperture) arranged between the monochromator 20 and a scintillator 28. The electrons 12b then collide with the scintillator 28, to which a high positive voltage is applied, generating light that is converted into an electrical signal by the photomultiplier 29 and amplified so that the signal can be viewed as a scanning electron microscopic image (SEM image). The auxiliary deflection device 25 can be adjusted axially with respect to the axis of the limiting aperture 27. Thus, by adjusting the diameter of the limiting aperture 27, only the electrons emitted at a specific desired solid angle to the optical axis can be detected and viewed as an SEM image.

[0024] Furthermore, by utilizing the energy decomposition caused by the deflection by the magnetic field generator 21, energy discrimination can be performed, for example, by electron energy loss spectroscopy (EELS) using an energy-sensitive analyzer 26 arranged between the auxiliary deflector 25 and the limiting aperture 27. Furthermore, to selectively detect electrons of a desired energy, the deflected electrons 12b can be scanned at the limiting aperture 27 using the auxiliary deflector 25 arranged between the magnetic field generator 21 and the aperture 27, and an image can be generated in predetermined energy units. This means that the electrons can be detected in units of the aforementioned material A, material B, and material C, so that the corresponding images can be generated.Furthermore, an operating device may be provided that determines, by performing the scanning operation, whether the detected quantity of electrons of a certain energy exceeds a predetermined threshold. For example, if a photoresist residue is present on the underside of the window and if the energy of the electrons emitted by the photoresist is known, a program for indicating the presence or absence of the residue based on whether the electrons with the energy are detected or whether an image of the photoresist residue is generated when the electrons with the energy are detected may be stored in a memory of the operating device (not shown).

[0025] The operating device also performs an operation to generate a composite image by combining the outputs from detector 16 and detector 29. For example, the image generated based on the electrons detected by detector 29 strongly reflects specific information about the composition of the window bottom, while the image generated based on the electrons detected by detector 16 strongly reflects information about the window top. Thus, if it is desired to observe the window bottom because information about the window top is not needed, image processing can be implemented to remove information about the window top, thereby enhancing information about the window bottom.

[0026] The apparatus according to the present embodiment is capable of capturing images or enhancing the information about material A, material B, and material C. Thus, the composition ratio can be varied in accordance with the purpose of measurement or inspection. The operation unit is equipped with a function for generating a line profile from the image and measuring the peak-to-peak distance in the profile. Thus, when it is desired to measure, for example, the dimension between material A and material C, the proportion of their information can be increased while the proportion of the remaining information can be relatively decreased when image composition is performed. In this way, the peak height for material A and material C can be increased, thereby improving measurement accuracy.

[0027] The yoke edge of the magnetic field generator 21 and the analyzer 26 are equipped with a convergence function to bring the deflected electrons 12b into convergence. The yoke edge surface of the magnetic field generator 21 is formed in a vertical plane relative to the input / output axis of the electrons, thereby providing a structure for suppressing the development of aberration contribution.

[0028] As stated above, it is possible to perform energy discrimination in addition to observing an SEM image, based on the electrons emitted from the sample that are within a desired solid angle, in a state of high resolution achieved by the monochromator and using a combination of the energy-sensitive analyzer 26 and the limiting aperture 27. The limiting aperture 27 may be movable so that the electrons of a desired energy can be selectively detected.

[0029] In the Fig. In the configuration shown in Figure 1, the magnetic field generator 21 of the monochromator 20 is used as a deflection device to guide the electrons emitted from the sample to the detector, as well as an energy separation unit to induce energy separation. Thus, the number of components arranged within a vacuum chamber can be reduced, simplifying the structure while simultaneously achieving high resolution and high energy separation of the detected electrons.

[0030] When the deflection angle of the magnetic field generator 21 changes due to the energy of the electrons 12b emitted from the sample and the voltage applied to the sample 11, the region from the auxiliary deflector 25 to the photomultiplier 29 may be configured to be movable according to the deflection angle, or a plurality of the regions may be arranged with respect to the yoke edge of the magnetic field generator 21 so that an SEM image can be observed at any deflection angle. Fig. Figure 2 illustrates an example in which a total of three such regions are arranged, one at the extension of the monochromator's symmetry plane and two above and below this plane. However, this is not a limitation; for example, two additional such regions can be provided on the left and right sides (perpendicular to the plane of the drawing), resulting in a total of five such regions. Alternatively, one or more CCDs and the like can be used for deflection.

[0031] An SEM image based on the electrons emitted from the desired solid angle can be observed from the SEM images from the photomultiplier 16 and the photomultiplier 28 using image processing.

[0032] Specifically, the conversion electrode 13 is provided with an opening for the passage of the primary electron beam 3, so that among the electrons emitted from the sample, the electrons 12a with angles exceeding a certain solid angle to the linear optical axis are caused to collide with the conversion electrode 13, thereby forming an SEM image using the photomultiplier 16. On the other hand, an SEM image of electrons 13b with all angles emitted from the sample is formed in the photomultiplier 29. By removing the information of the SEM image from the photomultiplier 16 from the SEM image from the photomultiplier 29 through image processing, only the information of the sample up to a certain solid angle to the linear optical axis can be obtained.The solid angle to the linear optical axis can be arbitrarily adjusted by changing the size of the opening of the conversion electrode 13 for the passage of the primary electron beam.

[0033] Fig.Figure 3 illustrates the relationship between the aperture angle (focal depth) of the primary electron beam incident on the sample and the resolution when the monochromator is used and when it is not used. By comparing the SEM image from the photomultiplier 16 (second detector) without using the monochromator and the SEM image from the photomultiplier 28 with the monochromator, but with the aperture angle adjusted so that the resolution is the same as without using the monochromator, a difference due to the difference in the focal depth can be indicated. The aperture angle can be adjusted by changing the opening diameter of the aperture 6 or by changing the convergence condition of the second convergence lens. In this case, the optical system control device 31 performs adjustment so that the magnification ratio does not change.

[0034] Although a low acceleration SEM was described above, similar operations can be performed with a high acceleration SEM, with the optical system or the conditions of the monochromator being adjusted by the electron microscope control device 30.

[0035] Although the scanning electron microscope equipped with the monochromator has been described above with reference to a specific embodiment, the present invention is not limited to this embodiment. It will be apparent to those skilled in the art that various modifications or improvements to the configuration or operation of the invention can be made in accordance with the above embodiment or other embodiments without departing from the scope of the present invention. List of reference symbols 1 electron source 2 anode electrode 3 Primary electron beam 3a Extended primary electron beam 4 First convergence lens 5 First astigmatism correction lens 6 aperture 7 Second convergence lens 8 Second astigmatism correction lens 9 Scanning deflection device 10 objective lens 11 Sample 12a, 12b, 12c, 12d Emitted electrons 13 Conversion electrode 14 Electrons generated by the conversion electrode 15, 28 Scintillator 16, 29 Photomultiplier 17 Acceleration electrode 18 Electron convergence lens 19, 25 Auxiliary deflection device 20 Monochromator 21 Magnetic field generator 22a First electric field generator 22b Second electric field generator 23 Energy selection aperture 24 plane of symmetry 26 Energy-sensitive analyzer 27 Limiting aperture 30 Electron microscope control device 31 Control device of the optical system 32 Monochromator control device

Claims

[1] A scanning electron microscope comprising an electron source (1) and a monochromator (20) which makes an electron beam (3) emitted by the electron source monochromatic, wherein: the monochromator comprises a magnetic field generator (21) which deflects the electron beam and an energy selection aperture (23) which passes a part of the electron beam deflected by the magnetic field generator; the magnetic field generator is arranged on a trajectory of electrons emitted by a sample (11) and is designed to deflect the electrons emitted by the sample onto different trajectories by means of a magnetic field generated by it when the sample is irradiated with the electron beam which has passed through the monochromator; and on each of the different trajectories, a first detector (29) and a first aperture (27) arranged between the magnetic field generator and the corresponding first detector are provided. [2] Scanning electron microscope according to claim 1, wherein on each of the different trajectories a deflection device (25) which deflects the electrons passed through the magnetic field generator (21) is arranged between the magnetic field generator and the corresponding first aperture (27). [3] A scanning electron microscope according to claim 1, comprising a second aperture (6) arranged between the magnetic field generator (21) and the sample (11) and limiting the passage of electrons emitted from the sample which are located in an angular range which is larger than a predetermined radiation angle with respect to an optical axis of the electron beam (3) as the center. [4] A scanning electron microscope according to claim 3, wherein the second aperture (6) is a conversion electrode which generates electrons upon collision with the electrons emitted by the sample (11), wherein the microscope further comprises a second detector (16) which detects the electrons emitted by the conversion electrode. [5] A scanning electron microscope according to claim 3, wherein the second aperture (6) is a second detector. [6] A scanning electron microscope according to claim 3, further comprising: a second detector (16) which detects the electrons collided with the second aperture (6) or the electrons produced by the collision with the second aperture; and an operating unit (31) which combines an output from the plurality of first detectors (29) and an output from the second detector. [7] A scanning electron microscope according to claim 1, further comprising an energy-sensitive analyzer (26) which analyzes an energy of the electrons emitted from the sample (11) and deflected by the magnetic field generator (21).

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