JP2016139607A - Scanning electron microscope - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a scanning electron microscope (SEM) enabling a surface analysis of a sample at a high throughput.SOLUTION: A SEM includes an electron gun, an irradiation unit, and a detector. The detector, as a first structure, includes an MCP, an anode, and a dynode. The dynode is set at a potential higher than a potential of an output face of the MCP and the anode is set at a potential higher than that of the dynode. The anode is disposed on the dynode side with respect to an intermediate position between the output face of the MCP and the dynode. The anode has an aperture for letting electrons from the output face of the MCP pass toward the dynode.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a scanning electron microscope (hereinafter referred to as “SEM”).

  For example, the SEM described in Patent Document 1 scans the electron beam irradiation position while irradiating a sample with a finely focused electron beam as a probe, and generates electrons (secondary electrons) generated at each position of the sample according to the electron beam irradiation. Electrons or backscattered electrons) are detected by a detector. With such electron detection, the SEM can perform surface analysis of the sample. The SEM has a feature that the enlargement ratio is larger than that of an optical microscope, and is indispensable as a device for visualizing nanoscale microstructures such as semiconductor mask pattern inspection.

  When the sample is irradiated with an electron beam, secondary electrons or reflected electrons are generated from the sample. Secondary electrons are electrons generated near the surface of the sample. An image (secondary electron image) obtained by detecting secondary electrons reflects fine irregularities of the sample. A reflected electron is an electron bounced off upon hitting an atom constituting the sample. Since the number of reflected electrons depends on the composition (average atomic number, crystal orientation, etc.) of the sample, the image (reflected electron image) obtained by detecting the reflected electrons reflects the composition distribution of the sample. Surface analysis of the sample can be performed based on the secondary electron image or the reflected electron image.

  Since the probe used in the SEM is an electron beam, it is not difficult to increase or decrease the electron dose applied to the sample surface. On the other hand, when measuring an insulator sample or the like, a charging phenomenon occurs and image quality is deteriorated. Therefore, in SEM, a quenching gas for limiting the amount of incident electrons and for preventing charging has been adopted so far.

JP 2007-42513 A

  As a result of examining the conventional SEM in detail, the inventors have found the following problems. That is, as a means for obtaining high image quality (high S / N) as a countermeasure against a decrease in input electrons and noise, it is important to increase the gain of the secondary electron detector. In recent years, due to the miniaturization of semiconductor mask patterns and nanoimprint technology, SEM is required to have a high throughput for processing fine pattern measurement at high speed. In response to this demand, attempts have been made to improve the measurement speed by reducing the electron beam diameter and sweeping the probe at high speed. As a result, a heavy burden is placed on the secondary electron detector, and the detector performance is becoming a bottleneck for high throughput.

  In SEM, a detector including a micro-channel plate (hereinafter referred to as “MCP”) is used. MCPs are widely used as secondary electron detectors for length-measuring scanning electron microscopes (CD-SEMs) because of their small size, light weight, high gain, and characteristics such as being less affected by magnetic fields. ing. In this specification, secondary electrons or reflected electrons that are generated on the sample surface and reach the input surface of the MCP are distinguished from secondary electrons that are multiplied in the MCP and output from the output surface of the MCP. Therefore, secondary electrons generated on the sample surface are simply referred to as “electrons”.

  In addition to MCP, there are semiconductors and photomultiplier tubes as detectors used in SEM. However, in order to detect secondary electrons with low energy, it is necessary to design the detector as close to the sample surface as possible and not disturb the electromagnetic field of the SEM column. Detectors other than MCP must be installed at a position away from the sample surface due to physical structure constraints, and therefore, an acceleration electric field must be added to collect secondary electrons with low energy. The electromagnetic field design becomes complicated. This is one of the main reasons why MCP is used as a detector for SEM.

  On the other hand, even in the SEM using the MCP detector, the throughput is not sufficient, and higher throughput is desired.

  The present invention has been made to solve the above-described problems, and provides a scanning electron microscope (SEM) having a structure for enabling surface analysis of a sample with high throughput. It is aimed.

  The SEM (scanning electron microscope) according to the present embodiment includes an electron gun, an irradiation unit, and a detector. The electron gun generates an electron beam. The irradiation unit irradiates the sample with an electron beam while scanning the electron beam irradiation position on the sample. The detector detects electrons generated in the sample in response to the electron beam irradiation on the sample. In particular, the detector includes an MCP (microchannel plate) that multiplies secondary electrons generated in response to incidence of electrons generated in the sample, a dynode, and a first structure configured by an anode, or MCP, It has the 2nd structure comprised with the anode and the electrode.

  In the detector having the first structure, the MCP has an input surface provided at a position where electrons from the sample reach and an output surface facing the input surface. The multiplied secondary electrons are output from the output surface. The dynode is provided on the opposite side of the MCP output surface to the MCP output surface, and multiplies secondary electrons output from the MCP output surface. The dynode is set to a potential higher than the potential of the output surface of the MCP. The anode is provided in a space from the dynode to the output surface of the MCP and an intermediate position between the dynodes in order to collect secondary electrons multiplied by the dynode. The anode has an opening through which secondary electrons output from the output surface of the MCP pass to the dynode. The anode is set to a potential higher than the potential of the dynode.

  On the other hand, in the detector having the second structure, the MCP has an input surface provided at a position where electrons from the sample reach and an output surface facing the input surface. The multiplied secondary electrons are output from the output surface. The anode is provided on the opposite side of the input surface of the MCP with respect to the output surface of the anode MCP in order to collect secondary electrons output from the MCP output surface. The anode is set to a potential higher than the potential of the output surface of the MCP. The electrode is provided in a space from the anode to the output surface of the MCP and an intermediate position between the anodes. This electrode has an opening through which secondary electrons output from the output surface of the MCP pass to the anode. Further, this electrode is set to a potential higher than the potential of the anode.

  Each embodiment according to the present invention can be more fully understood from the following detailed description and the accompanying drawings. These examples are given solely for the purpose of illustration and should not be considered as limiting the invention.

  Further scope of applicability of the present invention will become apparent from the detailed description given below. However, the detailed description and specific examples, while indicating the preferred embodiment of the invention, are presented for purposes of illustration only and various modifications and improvements within the scope of the invention may It will be apparent to those skilled in the art from the detailed description.

  According to this embodiment, it is possible to perform surface analysis of a sample with high throughput.

It is a figure which shows schematic structure of SEM (scanning electron microscope) which concerns on 1st Embodiment. It is a figure which shows each planar structure of a MCP laminated body, an anode, and a dynode with the cross-sectional structure of the detector which has 1st structure. It is a figure which shows the concrete structure for setting each electrode in the detector (1st structure) shown by FIG. 2 to predetermined electric potential, and the electric potential setting state in each electrode. It is a graph which shows the gain characteristic of the detector (1st structure) shown by FIG. It is a graph which shows the linearity characteristic of the detector (1st structure) shown by FIG. 3 is a graph showing a relationship between a dynode potential and a relative gain, measured while changing the aperture ratio of the anode in the detector (first structure) shown in FIG. 2. It is a figure which shows the cross-section of the detector which has a 2nd structure. It is a figure which shows the concrete structure for setting each electrode in the detector (2nd structure) shown by FIG. 7 to predetermined electric potential, and the electric potential setting state in each electrode. It is a graph which shows the linearity characteristic of the detector (2nd structure) shown by FIG.

[Description of Embodiment of Present Invention]
First, the contents of the embodiments of the present invention will be listed and described individually.

  (1) The SEM (scanning electron microscope) according to the present embodiment includes an electron gun, an irradiation unit, and a detector. The electron gun generates an electron beam. The irradiation unit irradiates the sample with an electron beam while scanning the electron beam irradiation position on the sample. The detector detects electrons generated in the sample in response to the electron beam irradiation on the sample. In particular, the detector includes an MCP (microchannel plate) that multiplies secondary electrons generated in response to incidence of electrons generated in the sample, a dynode, and a first structure configured by an anode, or MCP, It has the 2nd structure comprised with the anode and the electrode.

  In the detector having the first structure, the MCP has an input surface provided at a position where electrons from the sample reach and an output surface facing the input surface. The multiplied secondary electrons are output from the output surface. The dynode is provided on the opposite side of the MCP output surface to the MCP output surface, and multiplies secondary electrons output from the MCP output surface. The dynode is set to a potential higher than the potential of the output surface of the MCP. The anode is provided in a space from the dynode to the output surface of the MCP and an intermediate position between the dynodes in order to collect secondary electrons multiplied by the dynode. The anode has an opening through which secondary electrons output from the output surface of the MCP pass to the dynode. The anode is set to a potential higher than the potential of the dynode.

  On the other hand, in the detector having the second structure, the MCP has an input surface provided at a position where electrons from the sample reach and an output surface facing the input surface. The multiplied secondary electrons are output from the output surface. Since the anode collects secondary electrons output from the MCP output surface, the anode is provided on the opposite side of the MCP input surface with respect to the MCP output surface. The anode is set to a potential higher than the potential of the output surface of the MCP. The electrode is provided in a space from the anode to the output surface of the MCP and an intermediate position between the anodes. This electrode has an opening through which secondary electrons output from the output surface of the MCP pass to the anode. Further, this electrode is set to a potential higher than the potential of the anode.

  (2) In the detector having the first structure, the aperture ratio of the anode is preferably 90% or less as one aspect of the present embodiment. As one aspect of this embodiment, the anode preferably has a plurality of openings arranged two-dimensionally. Further, as one aspect of the present embodiment, the dynode is preferably made of a metal flat plate coated with a film that enhances secondary electron emission efficiency.

  (3) On the other hand, in the detector having the second structure, the aperture ratio of the electrode is preferably 90% or less as an embodiment of the present embodiment. As one aspect of this embodiment, the electrode preferably has a plurality of openings arranged two-dimensionally. As one aspect of this embodiment, the anode is preferably made of a metal flat plate.

  As described above, each aspect listed in this [Description of Embodiments of the Invention] is applicable to each of all the remaining aspects or to all combinations of these remaining aspects. .

[Details of the embodiment of the present invention]
Specific examples of the SEM according to the present invention will be described below in detail with reference to the accompanying drawings. It should be noted that the present invention is not limited to these exemplifications, is shown by the scope of the claims, and is intended to include all modifications within the meaning and scope equivalent to the scope of the claims. Yes. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted.

  First, how the present invention was conceived will be described. The MCP used as a detector in the SEM is a secondary electron multiplier having a plurality of microchannel structures that are two-dimensionally arranged and independent of each other. The MCP can perform electron multiplication by setting the output surface at a higher potential than the input surface. That is, when charged particles collide with the inner wall surface of each channel, secondary electrons are emitted, and the electrons are accelerated by the potential gradient and collide with the inner wall surface of the channel. Such a process is repeated in each channel, and a large number of multiplied electrons are output from the output surface.

  The electron multiplication function of the MCP is limited by charge saturation of the inner wall portion of each channel. In order to suppress this charge saturation, it is effective to supply electrons by a strip current flowing through the channel wall. Until now, attempts have been made to increase the strip current by lowering the resistance of the MCP. Enlarging the linear range of the extracted charge by reducing the resistance of the MCP is an effective means. However, on the other hand, the MCP resistor has a negative temperature coefficient, and since MCP is used in a high vacuum where heat dissipation is difficult, heat generation due to the strip current of the MCP itself causes warming and discharge phenomena. It has the problem that it ends up. The MCP detector used in the current SEM has a sufficiently low resistance measure, and it is practically difficult to reduce the resistance further.

Further, in order to convert electrons having only a monovalent elementary charge into an electric signal by a detector and to distinguish the electric signal from noise, a multiplication factor (gain) of about 10 ⁵ to 10 ⁶ is required. . Gain is essential to achieve high S / N. If the number of secondary electrons incident on the detector within a certain period of time increases significantly in order to meet the demand for higher throughput in spite of the longer scanning distance by narrowing the electron beam irradiating the sample, the MCP The S / N ratio is low due to the phenomenon of exceeding the upper limit of the linearity. That is, the maximum amount of incident electrons and the gain have a trade-off relationship due to the upper limit of linearity determined by the MCP resistance (the following equation (1)).
(MCP linearity upper limit) = (maximum amount of incident electrons) × (gain) (1)

  The present invention provides an SEM capable of performing surface analysis of a sample with high throughput based on the above-described studies by the present inventors, and is particularly characterized by the configuration of a detector. Hereinafter, embodiments of the SEM of the present invention will be described.

  FIG. 1 is a diagram showing a schematic configuration of a scanning electron microscope (SEM) 1 according to the present embodiment. The SEM 1 includes an electron gun 2, an anode 3, an alignment coil 4, a condenser lens 5, a deflection scanning coil 6, a backscattered electron detector 7, a secondary electron detector 8, an objective lens 9, a laser interferometer 10, a stage 11 and a motor 12. In a vacuum housing. Among these, the alignment coil 4, the condenser lens 5, the deflection scanning coil 6, and the objective lens 9 constitute an irradiation unit that scans the electron beam irradiation position on the sample 20 while irradiating the sample 20 with the electron beam.

  The electron gun 2 generates an electron beam. The electron gun 2 is made of, for example, a tungsten wire. The tip of the tungsten wire is shaped so as to be sharp and have a predetermined surface, and the electron gun 2 processed into such a shape generates an electron beam by a high voltage applied between the anode 3. The anode 3 is applied with a high voltage to the electron gun 2 to accelerate the electrons emitted from the electron gun 2. The alignment coil 4 is aligned by deflecting the electron beam emitted from the electron gun 2. The condenser lens 5 focuses the electron beam emitted from the electron gun 2. The deflection scanning coil 6 scans the electron beam irradiated on the sample 20 in the X direction and the Y direction.

  The backscattered electron detector 7 detects backscattered electrons generated in the sample 20 when the sample 20 is irradiated with an electron beam. The backscattered electron detector 7 has a disk shape and has a hole through which an electron beam passes in the center, and has a fan shape in which the entire circumference is divided into, for example, four parts. A reflected electron image having light and shade can be obtained, and the unevenness of the shape can be highlighted by the difference between the two left and right signals. The secondary electron detector 8 collects, detects, and amplifies secondary electrons generated in the sample 20 when the sample 20 is irradiated with an electron beam. The secondary electron detector 8 is disk-shaped and has a hole through which an electron beam passes. A secondary electron image can be obtained based on the signal detected by the secondary electron detector 8.

  The objective lens 9 narrows the electron beam thinly on the surface of the sample 20. The laser interferometer 10 measures the position of the stage 11 by causing the laser beam output toward the reflecting mirror attached to the stage 11 to interfere with the laser beam reflected by the reflecting mirror. The stage 11 is a device that places the sample 20 and moves the sample 20 to an arbitrary coordinate position accurately, and is driven by a motor 12. The motor 12 is power for driving the stage 11 in the X direction and the Y direction, and a servo motor, an ultrasonic motor, or the like is suitable.

  FIGS. 2A and 3A are diagrams showing a configuration of a detector 100A applicable to the SEM of FIG. This detector 100A is used as the secondary electron detector 8 in FIG. The detector 100A may be used as the backscattered electron detector 7. The detector 100A includes, as a first structure, a stacked body composed of MCP 111 and MCP 112 (hereinafter referred to as “MCP stacked body”), an anode 120A, a dynode 130A, and a bleeder circuit 200A connected to an external power source 300A. The bleeder circuit 200A applies a predetermined voltage to each electrode in order to form a potential gradient as in the example shown in FIG.

  In the detector 100A having the first structure, each of the MCPs 111 and 112 is a secondary electron multiplier having a plurality of microchannel structures that are two-dimensionally arranged and independent of each other. Each channel has an inner diameter of about 10 μm and is inclined about 10 ° with respect to the vertical direction (coincidence with the electron incident direction) of the input surface of the MCP laminate (hereinafter referred to as “MCP input surface”). However, the MCP 111 and the MCP 112 have different channel tilt directions. A lead wire 114 extending from the bleeder circuit 200A is connected to the MCP input surface via an in-electrode (hereinafter referred to as “MCP-IN electrode”) 113. Similarly, an output surface (hereinafter referred to as an MCP laminated body). A lead wire 116 extending from the bleeder circuit 200A is connected to the “MCP output surface” via an out electrode (hereinafter referred to as “MCP-OUT electrode”) 115. That is, when a predetermined voltage is applied from the bleeder circuit 200A to the MCP-IN electrode 113 and the MCP-OUT electrode 115 via the lead wires 114 and 116, the MCP input surface and the MCP output surface are set to predetermined potentials. The In the MCP laminate, the output surface is set to a high potential with respect to the input surface, thereby multiplying secondary electrons generated in response to the arrival of electrons at the input surface and multiplying from the output surface. Outputs electrons.

The dynode 130A is provided on the side of the MCP output surface (on the side opposite to the MCP input surface with respect to the MCP output surface), and multiplies secondary electrons output from the MCP output surface. A bleeder circuit 200A is connected to the dynode 130A via a lead wire 131A. When a predetermined voltage is applied from the bleeder circuit 200A to the dynode 130A, the potential of the dynode 130A is set higher than the MCP output surface. The The dynode 130A is a metal flat plate (for example, a SUS flat plate) disposed in parallel to the MCP output surface. In the dynode 130A, the surface of the metal flat plate (the surface facing the MCP output surface) is preferably coated with a high δ film (a film having a high secondary electron emission efficiency). The high δ film is, for example, an alkali metal film, preferably an MgF ₂ film.

  The anode 120A is provided in parallel to the MCP output surface in a space from the dynode 130A to the intermediate position between the MCP output surface and the dynode 130A. The anode 120A may be disposed at an intermediate position between the MCP output surface and the dynode 130A. The anode 120A has an opening through which secondary electrons output from the MCP output surface pass to the dynode 130A. The anode 120A is connected to the lead wire 121A, and the electric pulse signal output from the anode 120A is amplified by an amplifier (Amp) 250. Note that a capacitor 251 is disposed between the anode 120A and the amplifier 250, and this capacitor 251 has a function of setting the signal output level to the ground level by insulating the output. The potential of the anode 120A is set higher than that of the dynode 130A, and secondary electrons multiplied by the dynode 130A are collected. The aperture ratio of the anode 120A is preferably 90% or less. The anode 120A preferably has a mesh shape having a plurality of openings arranged two-dimensionally.

  A hole is provided in the center of each of the MCP laminate (FIG. 2 (b)), the anode 120A (FIG. 2 (c)), and the dynode 130A (FIG. 2 (d)). 160 is inserted. The electron beam to be irradiated on the sample 20 passes through the internal space of the tube portion 160.

  The anode 120A is sandwiched between the ceramic plate 141 and the ceramic plate 142. The dynode 130A is sandwiched between the ceramic plate 142 and the ceramic plate 143. Each of the MCP-IN electrode 113, the MCP-OUT electrode 115, and the ceramic plates 141 to 143 has an annular shape. The relative positional relationship between the MCP-IN electrode 113, the MCP-OUT electrode 115, and the ceramic plates 141 to 143 is fixed by screws 151 and 152, thereby assembling the detector 100A having the first structure.

  In this detector 100A, an anode 120A and a dynode 130A are arranged in order along the direction from the MCP input surface to the MCP output surface. Further, the bleeder circuit 200A is connected to the lead wire 114 (FIGS. 2A and 3A) so that the potential of the dynode 130A is higher than the potential of the MCP output surface and the potential of the anode 120A is higher than the potential of the dynode 130A. ) And ground potential in the example of FIG. 3B), a predetermined voltage is applied to these electrodes via 116, 121A and 131A (see FIG. 3A and FIG. 3B). When electrons reach the MCP input surface, secondary electrons generated in response to the arrival of the electrons are multiplied in the MCPs 111 and 112. The multiplied secondary electrons are output from the MCP output surface. Most of the many secondary electrons output from the MCP output surface pass through the opening of the anode 120A and collide with the dynode 130A. Due to this collision, a larger number of secondary electrons are generated at the dynode 130A. A large number of secondary electrons generated in the dynode 130A are collected by the anode 120A. That is, when electrons reach the MCP input surface, an electric pulse signal having a peak value corresponding to the number of electrons is output from the anode 120A.

  An example of the potential gradient shown in FIG. 3B is that the potential V1 of the MCP input surface (MCP-IN electrode 113) is 0 V (ground potential), the potential V2 of the MCP output surface MCP-OUT electrode 115) is +2000 V, The potential V3 of the anode 120A is set to + 2500V, and the potential V4 of the dynode 130A is set to a positive potential within the range of V2 to V3 (V4 setting range). Note that the potential gradient from the MCP-IN electrode 113 to the anode 120A may be set such that the potential V3 of the anode 120A is 0 V (ground potential) as shown in the following experimental example. In this case, for example, the potential V1 of the MCP-IN electrode 113 is set to -2500V, the potential V2 of the MCP-OUT electrode 115 is set to -500V, the potential V3 of the anode 120A is set to 0V (ground potential), and the potential V4 of the dynode 130A is set. Is set to a negative potential within the range of V2 to V3 (V4 setting range).

FIG. 4 is a graph showing gain characteristics of the detector 100A. The horizontal axis represents gain, and the vertical axis represents the count value (pulse count) of electrons output from the MCP output surface. In both the detector 100A having the first structure and the comparative example, the distance between the MCP output surface and the anode 120A is 1 mm, and the distance between the anode 120A and the dynode 130A is 1 mm. The dynode 130A is a SUS plate coated with an MgF ₂ film. The potential V1 of the MCP input surface is -2500V, the potential V2 of the MCP output surface is -500V, and the potential V3 of the anode 120A is 0V (ground potential). In the comparative example, the potential V4 of the dynode 130A was set to 0 V (ground potential), and the anode 120A and the dynode 130A were bundled to detect all output secondary electrons from the MCP. In the detector 100A applied to the present embodiment, secondary electrons are detected at the anode 120A when the potential V4 of the dynode 130A is set to −250V.

  As can be seen from FIG. 4, the gain of the detector 100A was about 6.3 times the gain of the comparative example. In the gain characteristic of the detector 100A, a sub peak is observed at the gain peak position of the comparative example. This is because a part of a large number of secondary electrons output from the MCP output surface does not reach the dynode 130A, but directly by the anode 120A. Indicates that it was captured. In the following, the detector 100A (potential of the anode 120A rather than the potential V4 of the dynode 130A) with respect to the gain of the comparative example (when the anode 120A and the dynode 130A are bundled and the anode 120A and the dynode 130A are set to the same potential). The gain ratio when V3 is set high is referred to as “relative gain”.

FIG. 5 is a graph showing the linearity characteristics of the detector 100A. The horizontal axis is the output current value (A) from the anode 120A, and the vertical axis is the normalized gain. The normalized gain is set to 100 when the output current value is small. In FIG. 5, the symbol “●” indicates the linearity characteristic when the potential V4 of the dynode 130A is set to the same potential as the potential V3 of the anode 120A, and the symbol “■” indicates that the potential V4 of the dynode 130A is the potential V3 of the anode 120A. Linearity characteristics when symbol is set to −100V, symbol “♦” is linearity characteristics when potential V4 of dynode 130A is set to −200V with respect to potential V3 of anode 120A, symbol “▲” is dynode The linearity characteristic when the potential V4 of 130A is set to −300V with respect to the potential V3 of the anode 120A, and the combined symbol “*” and “−” indicates that the potential V4 of the dynode 130A is − with respect to the potential V3 of the anode 120A. The linearity characteristic when the voltage is set to 400 V, the symbol “×” indicates the Indicating position V4 is the linearity characteristic when set to -500V with respect to the potential V3 of the anode 120A, respectively. In both the detector 100A and the comparative example used for this measurement, the distance between the MCP output surface and the anode 120A is 1 mm, and the distance between the anode 120A and the dynode 130A is 1 mm. The dynode 130A is a SUS plate coated with an MgF ₂ film. The potential V1 of the MCP input surface was -2500V, the potential V2 of the MCP output surface was -500V, and the potential V3 of the anode 120A was 0V (ground potential). In the comparative example, the potential V4 of the dynode 130A was set to 0 V, and the dynode 130A and the anode 120A were bundled. As can be seen from FIG. 5, in the detector 100A in which the potential V4 of the dynode 130A is set to −200 V with respect to the potential V3 of the anode 120A, the DC linearity is increased by about 7 times compared to the comparative example.

  4 and 5, it can be seen that the linearity of the detector 100A applied to this embodiment with respect to the comparative example is increased by the gain multiplication.

  FIG. 6 is a graph showing the relationship between the dynode potential V4 of the detector 100A and the relative gain, measured while changing the aperture ratio of the anode 120A. FIG. 6A shows the relationship when the aperture ratio of the anode 120A is 81%. FIG. 6B shows the relationship when the aperture ratio of the anode 120A is 90%. FIG. 6C shows the relationship when the aperture ratio of the anode 120A is 96%. In the detector 100A used for this measurement, the dynode 130A is a SUS plate not coated with a high δ film. The potential V1 of the MCP input surface is -2500V, the potential V2 of the MCP output surface is -500V, and the potential V3 of the anode 120A is 0V (ground potential). The potential range of the changed dynode 130A is −50V to −500V. 6 (a) to 6 (c), the distance between the MCP output surface and the dynode 130A is set to 2.0 mm, the distance d1 between the MCP output surface and the anode 120A, and the anode 120A The measured values in the configuration in which the ratio d1 / d2 of the distance d2 to the dynode 130A is set to 0.5 mm / 1.5 mm, 1.0 mm / 1.0 mm, and 1.5 mm / 0.5 mm, respectively. Has been.

  As can be seen from FIGS. 6A to 6C, the relative gain is larger when the distance d2 is 1.0 mm than when the distance d2 between the anode 120A and the dynode 130A is 1.5 mm. When the distance d2 is 0.5 mm, the relative gain is even larger. Therefore, when the anode 120A is disposed in a space from the dynode 130A to the intermediate position between the MCP output surface and the dynode 130A (the anode 120A may be disposed at an intermediate position between the MCP output surface and the dynode 130A), the relative gain is set. Can be increased. The relative gain difference is significant when the potential difference between the anode 120A and the dynode 130A is small, and is significant when the aperture ratio of the anode 120A is small. Therefore, the aperture ratio of the anode 120A is preferably 90% or less.

  Next, a detector 100B having a second structure that can be applied to the SEM 1 in FIG. 1 will be described with reference to FIGS. 7 (a), 8 (a) to 8 (b), and FIG. FIGS. 7A and 8A are diagrams showing a configuration of a detector 100B applicable to the secondary electron detector 8 (or the backscattered electron detector 7) of the SEM 1 in FIG. This detector 100B includes, as a second structure, an MCP laminate including MCP 111 and MCP 112, an anode 120B, an electrode 130B, and a bleeder circuit 200B connected to an external power source 300B. The bleeder circuit 200B applies a predetermined voltage to each electrode in order to form a potential gradient like the example shown in FIG.

  In the detector 100B having the second structure, each of the MCPs 111 and 112 is a secondary electron multiplier having a plurality of microchannel structures that are two-dimensionally arranged and independent of each other. Each channel has an inner diameter of about 10 μm and is inclined about 10 ° with respect to the vertical direction of the MCP input surface. However, the MCP 111 and the MCP 112 have different channel tilt directions. A lead wire 114 extending from the bleeder circuit 200B is connected to the MCP input surface via the MCP-IN electrode 113. Similarly, a lead wire 116 extending from the bleeder circuit 200B is connected to the MCP output surface via the MCP-OUT electrode 115. That is, when a predetermined voltage is applied from the bleeder circuit 200B to the MCP-IN electrode 113 and the MCP-OUT electrode 115 via the lead wires 114 and 116, the MCP input surface and the MCP output surface are set to predetermined potentials. The In the MCP laminate, the output surface is set to a high potential with respect to the input surface, thereby multiplying secondary electrons generated in response to the arrival of electrons at the input surface and multiplying from the output surface. Outputs electrons.

  The anode 120B is provided on the side of the MCP output surface (the side opposite to the MCP input surface with respect to the MCP output surface). A bleeder circuit 200B is connected to the anode 120B via a lead wire 121B. When a predetermined voltage is applied from the bleeder circuit 200B to the anode 120B, the potential of the anode 120B is set to a potential higher than that of the MCP output surface. The The anode 120B is a metal flat plate (for example, a SUS flat plate) arranged in parallel to the MCP output surface, and collects secondary electrons output from the MCP output surface by being set at a higher potential than the MCP output surface. The electric pulse signal output from the anode 120B is amplified by an amplifier (Amp) 250. Note that a capacitor 251 is disposed between the anode 120B and the amplifier 250, and this capacitor 251 has a function of setting the signal output level to the ground level by insulating the output.

  The electrode 130B is provided in parallel to the MCP output surface in a space from the anode 120B to the intermediate position between the MCP output surface and the anode 120B. The electrode 130B may be disposed at an intermediate position between the MCP output surface and the anode 120B. The electrode 130B has an opening through which the secondary electrons output from the MCP output surface pass to the anode 120B. The electrode 130B is connected to the lead wire 131B, and the potential of the electrode 130B is set higher than that of the anode 120B. The aperture ratio of the electrode 130B is preferably 90% or less. The electrode 130B preferably has a mesh shape having a plurality of openings arranged two-dimensionally.

  A hole is provided in the center of each of the MCP laminate (FIG. 7 (b)), the electrode 130B (FIG. 7 (c)) and the anode 120B (FIG. 7 (d)). 160 is inserted. The electron beam to be irradiated on the sample 20 passes through the internal space of the tube portion 160.

  The electrode 130B is sandwiched between the ceramic plate 141 and the ceramic plate 142. The anode 120B is sandwiched between the ceramic plate 142 and the ceramic plate 143. Each of the MCP-IN electrode 113, the MCP-OUT electrode 115, and the ceramic plates 141 to 143 has an annular shape. The relative positional relationship among the MCP-IN electrode 113, the MCP-OUT electrode 115, and the ceramic plates 141 to 143 is fixed by screws 151 and 152, thereby assembling the detector 100B having the second structure.

  In this detector 100B, an electrode 130B and an anode 120B are arranged in order along the direction from the MCP input surface to the MCP output surface. The bleeder circuit 200B is connected to the lead wire 114 (FIGS. 7A and 8A) so that the potential of the anode 120B is higher than the potential of the MCP output surface and the potential of the electrode 130B is higher than the potential of the anode 120B. ) And the ground potential in the example of FIG. 8 (b)), 116, 121B, 131B (positive potential in the examples of FIG. 7 (a), FIG. 8 (a) and FIG. 8 (b)). A voltage is applied (see FIGS. 8A and 8B). When electrons reach the MCP input surface, secondary electrons generated in response to the arrival of the electrons are multiplied in the MCPs 111 and 112. The multiplied secondary electrons are output from the MCP output surface and accelerated toward the anode 120B by the electrode 130B. As a result, most of the many secondary electrons output from the MCP output surface pass through the opening of the electrode 130B and are collected by the anode 120B. That is, when electrons reach the MCP input surface, an electric pulse signal having a peak value corresponding to the number of electrons is output from the anode 120B.

  An example of the potential gradient shown in FIG. 8B is that the potential V1 of the MCP input surface (MCP-IN electrode 113) is 0V (ground potential), and the potential V2 of the MCP output surface (MCP-OUT electrode 115) is + 2000V. The potential V3 of the anode 120B is set to + 2100V, and the potential V4 of the electrode 130B is set to a positive potential (for example, + 2500V) in a range exceeding V2 (V4 setting range). Note that the potential gradient from the MCP-IN electrode 113 to the anode 120B may be set such that the potential V3 of the anode 120B is 0 V (ground potential) as shown in the following experimental example. In this case, for example, the potential V1 of the MCP-IN electrode 113 is set to -2300V, the potential V2 of the MCP-OUT electrode 115 is set to -500V, the potential V3 of the anode 120B is set to 0V (ground potential), and the potential V4 of the electrode 130B is set. Is set to a positive potential (for example, +500 V) in a range exceeding V3 (V4 setting range).

  FIG. 9 is a graph showing the linearity characteristic of the detector 100B. The horizontal axis is the output current value (A) from the anode 120B, and the vertical axis is the normalized gain. The normalized gain is set to 100 when the output current value is small. In FIG. 9, the symbol “♦” indicates linearity characteristics when the potential V4 of the electrode 130B is set to the same potential as the potential V3 of the anode 120B, and the symbol “■” indicates that the potential V4 of the electrode 130B is the potential V3 of the anode 120B. The linearity characteristic when the voltage is set to + 100V, symbol “▲” is the linearity characteristic when the potential V4 of the electrode 130B is set to + 200V with respect to the potential V3 of the anode 120B, and the symbol “×” is the symbol of the electrode 130B. The linearity characteristic when the potential V4 is set to +300 V with respect to the potential V3 of the anode 120B, and the symbol “*” denotes the linearity characteristic when the potential V4 of the electrode 130B is set to +400 V with respect to the potential V3 of the anode 120B. Each is shown. In the detector 100B used for this measurement, the distance between the MCP output surface and the electrode 130B is 1 mm, and the distance between the electrode 130B and the anode 120B is 1 mm. The anode 120B is a SUS plate. The potential V1 of the MCP input surface was -2300V, the potential V2 of the MCP output surface was -1500V, and the potential V3 of the anode 120B was 0V (ground potential). As can be seen from FIG. 9, the DC linearity is expanded by securing a potential difference between the electrode 130B and the anode 120B of, for example, 200 V or more even with the detector 100B having the second structure. Further, it can be seen that the linearity of the detector 100B applied to the present embodiment as compared with the comparative example of FIG.

  The detector 100A or the detector 100B having the above-described structure is applied to the detector 8 (or the detector 7) of the SEM of the present embodiment. Therefore, even if the amount of electrons incident on the detector 100A or the detector 100B increases, the gain of the entire detector can be increased while suppressing an increase in the gain of the MCPs 111 and 112. Therefore, the SEM 1 of this embodiment can perform surface analysis of a sample with high throughput. In addition, since both the detector 100A and the detector 100B can keep the gain of the MCPs 111 and 112 low, the voltage applied between the input surface and the output surface of the MCP laminate can be lowered, and the lifetime Improved characteristics. The detector 100A has a configuration in which an anode 120A is inserted between the MCP stack and the dynode 130A, and the detector 100B has a configuration in which an electrode 130B is inserted between the MCP stack and the anode 120B. Therefore, the increase in size is suppressed as compared with the conventional configuration.

  From the above description of the present invention, it is apparent that the present invention can be modified in various ways. Such modifications cannot be construed as departing from the spirit and scope of the invention, and modifications obvious to one skilled in the art are intended to be included within the scope of the following claims.

  DESCRIPTION OF SYMBOLS 1 ... Scanning electron microscope (SEM), 2 ... Electron gun, 3 ... Anode, 4 ... Alignment coil, 5 ... Condenser lens, 6 ... Deflection scanning coil, 7 ... Reflection electron detector, 8 ... Secondary electron detector, DESCRIPTION OF SYMBOLS 9 ... Objective lens, 10 ... Laser interferometer, 11 ... Stage, 12 ... Motor, 20 ... Sample, 100A, 100B ... Detector, 111, 112 ... Microchannel plate (MCP), 113 ... MCP-IN electrode, 115 ... MCP-OUT electrode, 120A, 120B ... anode, 130A ... dynode, 130B ... electrode.

Claims (8)

An electron gun that generates an electron beam;
An irradiation unit that irradiates the sample with the electron beam while scanning an electron beam irradiation position on the sample;
A detector that detects electrons generated in the sample in response to electron beam irradiation on the sample,
The detector is
A microchannel plate for multiplying secondary electrons generated in response to incidence of electrons generated in the sample, the input surface being provided at a position where electrons from the sample reach, and facing the input surface A microchannel plate having an output surface for outputting the multiplied secondary electrons,
A dynode provided on the opposite side of the input surface with respect to the output surface for multiplying secondary electrons output from the output surface, the dynode being set to a potential higher than the potential of the output surface When,
An anode that collects secondary electrons multiplied by the dynode and is provided in a space from the dynode to the output surface and an intermediate position between the dynodes and output from the output surface. And an anode set to a potential higher than the potential of the dynode.   The scanning electron microscope according to claim 1, wherein an aperture ratio of the anode is 90% or less.   The scanning electron microscope according to claim 1, wherein the anode has a plurality of openings arranged two-dimensionally.   4. The scanning electron microscope according to claim 1, wherein the dynode is formed of a metal flat plate coated with a film for increasing secondary electron emission efficiency. 5. An electron gun that generates an electron beam;
An irradiation unit that irradiates the sample with the electron beam while scanning an electron beam irradiation position on the sample;
A detector that detects electrons generated in the sample in response to electron beam irradiation on the sample,
The detector is
A microchannel plate for multiplying secondary electrons generated in response to incidence of electrons generated in the sample, the input surface being provided at a position where electrons from the sample reach, and facing the input surface A microchannel plate having an output surface for outputting the multiplied secondary electrons,
An anode that is provided on the opposite side of the input surface with respect to the output surface and collects secondary electrons output from the output surface, the anode being set to a potential higher than the potential of the output surface; ,
An electrode provided in a space from the anode to an intermediate position between the output surface and the anode, and having an opening for allowing secondary electrons output from the output surface to pass to the anode, And a scanning electron microscope including an electrode set to a potential higher than the potential.   The scanning electron microscope according to claim 5, wherein an aperture ratio of the electrode is 90% or less.   The scanning electron microscope according to claim 5 or 6, wherein the electrode has a plurality of openings arranged two-dimensionally.   The scanning electron microscope according to claim 5, wherein the anode is made of a metal flat plate.