Scanning electron microscope

An embodiment of the invention relates to a SEM enabling a surface analysis of a sample at a high throughput. The SEM has 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.

TECHNICAL FIELD

The present invention relates to a Scanning Electron Microscope (hereinafter referred to as “SEM”).

BACKGROUND

For example, the SEM described in Japanese Unexamined Patent Publication No. 2007-42513 (Patent Literature 1) is configured to scan a sample by irradiating the sample with a narrowly-focused electron beam as probe and moving the position of irradiation with the electron beam, and to detect electrons (secondary electrons or reflected electrons) generated at each position of the sample in accordance with the irradiation with the electron beam, by a detector. The SEM can perform a surface analysis of the sample by such electron detection. SEM has the feature of larger magnifying powers than optical microscopes and is indispensable as apparatus for visualizing nanoscale microstructures, e.g., for inspection of semiconductor mask patterns.

When a sample is irradiated with the electron beam, the sample generates secondary electrons or reflected electrons. The secondary electrons are electrons generated from near the surface of the sample. An image obtained by detection of secondary electrons (secondary electron image) reflects microscopic unevenness of the sample. The reflected electrons are electrons recoiling upon collision with atoms constituting the sample. Since the number of reflected electrons depends on a composition of the sample (an average atomic number, crystallographic orientation, or the like), an image obtained by detection of reflected electrons (reflected electron image) reflects a composition distribution of the sample. The surface analysis of the sample can be implemented based on the secondary electron image or the reflected electron image.

Since the probe used in SEM is the electron beam, it is not difficult to increase or decrease an electron dose of the electron beam irradiating the surface of the sample. On the other hand, in measuring an insulator sample or the like, a charging phenomenon occurs, resulting in degradation of image quality. For this reason, SEM has been used heretofore while adopting countermeasures such as limitation of the incident electron amount or a quenching gas for prevention of charging.

SUMMARY

The Inventors conducted detailed research on the conventional SEMs and found the problem as described below. Namely, increase in gain of the secondary electron detector becomes important as means for gaining high quality (high S/N), with decrease in incident electrons and suppression of noise. With miniaturization of semiconductor mask patterns and development of nanoimprint technology, SEM is needed in recent years to achieve a higher throughput to process measurement of microscopic pattern at high speed. For meeting this need, there is an attempt to decrease the diameter of the electron beam and sweep the probe at high speed, thereby increasing the measurement speed. As a result, a great burden is imposed on the secondary electron detector and the performance of the detector is becoming a bottle neck to achievement of a higher throughput.

SEM uses the detector including a Micro-Channel Plate (hereinafter referred to as “MCP”). MCP has characteristics such as small size, light weight, high gain, and being little affected by a magnetic field; therefore, MCP is commonly used as a secondary electron detector for Critical Dimension-Scanning Electron Microscope (CD-SEM) or the like. In the present specification, the secondary electrons generated from near the surface of the sample will be simply referred to as “electrons,” in order to discriminate the secondary electrons or reflected electrons generated from near the surface of the sample and reaching an input face of an MCP from secondary electrons multiplied in the MCP and outputted from an output face of the MCP.

Detectors used in SEM include semiconductor detectors and photomultipliers as well as MCP. For detecting secondary electrons with low energy, however, it is necessary to adopt a layout wherein the detector is set as close to the sample surface as possible, without disturbance of an electromagnetic field of a SEM cylinder. The detectors except for MCP have to be installed at a position distant from the sample surface because of restrictions of physical structure and, for this reason, addition of an acceleration electric filed is needed to collect secondary electrons with low energy, so as to make design of the electromagnetic field complicated. This is one of big reasons for use of MCP as a detector of SEM.

On the other hand, the conventional SEMs using the MCP detector had unsatisfactory throughputs and there are desires for achievement of a higher throughput.

The present invention has been accomplished in order to solve the above-described problem and it is an object of the present invention to provide a scanning electron microscope (SEM) with a structure for enabling the surface analysis of the sample at a high throughput.

A SEM (scanning electron microscope) according to an embodiment of the invention has an electron gun, an irradiation unit, and a detector. The electron gun generates an electron beam. The irradiation unit irradiates a sample with the electron beam while scanning the sample by moving a position of irradiation with the electron beam on the sample. The detector detects electrons generated from the sample in accordance with the irradiation of the sample with the electron beam. Particularly, the detector has a first structure comprised of an MCP (micro-channel plate) for multiplying secondary electrons generated in accordance with incidence of the electrons generated from the sample, a dynode, and an anode, or, a second structure comprised of an MCP, an anode, and an electrode.

In the detector having the first structure, the MCP has an input face located at a position of arrival of the electrons from the sample, and an output face opposing the input face. The multiplied secondary electrons are outputted from the output face. The dynode is disposed on the opposite side to the input face of the MCP with respect to the output face of the MCP and multiplies the secondary electrons outputted from the output face of the MCP. The dynode is set at a potential higher than a potential of the output face of the MCP. The anode is disposed in a space from the dynode to an intermediate position between the output face of the MCP and the dynode, in order to collect the secondary electrons multiplied by the dynode. The anode has an aperture for letting the secondary electrons outputted from the output face of the MCP, pass toward the dynode. Furthermore, the anode is set at 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 face located at a position of arrival of the electrons from the sample, and an output face opposing the input face. The multiplied secondary electrons are outputted from the output face. The anode is disposed on the opposite side to the input face of the MCP with respect to the output face of the MCP, in order to collect the secondary electrons outputted from the output face of the MCP. The anode is set at a potential higher than a potential of the output face of the MCP. The electrode is disposed in a space from the anode to an intermediate position between the output face of the MCP and the anode. This electrode has an aperture for letting the secondary electrons outputted from the output face of the MCP, pass toward the anode. Furthermore, this electrode is set at a potential higher than the potential of the anode.

Each of embodiments according to the present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings. These examples are presented by way of illustration only, and thus are not to be considered as limiting the present invention.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, and it is apparent that various modifications and improvements within the scope of the invention would be obvious to those skilled in the art from this detailed description.

DETAILED DESCRIPTION

Description of Embodiment of Invention

First, the contents of the embodiment of the invention will be described each as individually enumerated below.

(1) A SEM (scanning electron microscope) according to the embodiment of the invention has an electron gun, an irradiation unit, and a detector. The electron gun generates an electron beam. The irradiation unit irradiates a sample with the electron beam while scanning the sample by moving a position of irradiation with the electron beam on the sample. The detector detects electrons generated from the sample in accordance with the irradiation of the sample with the electron beam. Particularly, the detector has a first structure comprised of an MCP (micro-channel plate) for multiplying secondary electrons generated in accordance with incidence of the electrons generated from the sample, a dynode, and an anode, or, a second structure comprised of an MCP, an anode, and an electrode.

In the detector having the first structure, the MCP has an input face located at a position of arrival of the electrons from the sample, and an output face opposing the input face. The multiplied secondary electrons are outputted from the output face. The dynode is disposed on the opposite side to the input face of the MCP with respect to the output face of the MCP and multiplies the secondary electrons outputted from the output face of the MCP. The dynode is set at a potential higher than a potential of the output face of the MCP. The anode is disposed in a space from the dynode to an intermediate position between the output face of the MCP and the dynode, in order to collect the secondary electrons multiplied by the dynode. The anode has an aperture for letting the secondary electrons outputted from the output face of the MCP, pass toward the dynode. Furthermore, the anode is set at 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 face located at a position of arrival of the electrons from the sample, and an output face opposing the input face. The multiplied secondary electrons are outputted from the output face. The anode is disposed on the opposite side to the input face of the MCP with respect to the output face of the MCP, in order to collect the secondary electrons outputted from the output face of the MCP. The anode is set at a potential higher than a potential of the output face of the MCP. The electrode is disposed in a space from the anode to an intermediate position between the output face of the MCP and the anode. This electrode has an aperture for letting the secondary electrons outputted from the output face of the MCP, pass toward the anode. Furthermore, this electrode is set at a potential higher than the potential of the anode.

(2) In the detector having the first structure, as one aspect of the embodiment, an aperture rate of the anode is preferably not more than 90%. As one aspect of the embodiment, the anode preferably has a plurality of apertures arranged two-dimensionally. Furthermore, as one aspect of the embodiment, the dynode is preferably comprised of a metal flat plate coated with a film to increase a secondary electron emission efficiency.

(3) On the other hand, in the detector having the second structure, as one aspect of the embodiment, an aperture rate of the electrode is preferably not more than 90%. As one aspect of the embodiment, the electrode preferably has a plurality of apertures arranged two-dimensionally. As one aspect of the embodiment, the anode is preferably comprised of a metal flat plate.

Each of the aspects enumerated in this [Description of Embodiment of Invention] above is applicable to each of all the remaining aspects or to all combinations of these remaining aspects.

Details of Embodiment of 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 by no means intended to be limited to these examples presented by way of illustration but is intended for inclusion of all changes within the meaning and scope of equivalence to the scope of claims, as described in the scope of claims. The same elements will be denoted by the same reference signs in the description of the drawings, without redundant description.

First described is how we have accomplished the present invention. The MCP used as a detector in the SEM is a secondary electron multiplier having a structure with a plurality of micro-channels arranged two-dimensionally and independently of each other. The MCP has an output face set at a higher potential than an input face whereby it can multiply electrons. Specifically, when a charged particle collides with an inner wall face of each channel, secondary electrons are emitted therefrom and the electrons are accelerated by a potential gradient to collide with the inner wall face of the channel. This process is repeated in each channel, whereby a large number of multiplied electrons are outputted from the output face.

The electron multiplication function of MCP is restricted when the inner wall of each channel is saturated with charge. For restraining this charge saturation, it is effective to supply electrons by strip current flowing in the channel wall. There was the conventional attempt to increase the strip current by reduction in resistance of the MCP. Expansion of the linear range (linearity) of extracted charge by reduction in resistance of the MCP is an effective means. On the other hand, however, the MCP resistance has a negative temperature coefficient and the MCP is used in a high vacuum where radiation of heat is difficult; for this reason, it has the problem that generation of heat by the strip current in the MCP itself can cause warming and discharge phenomena. Since the MCP detector used in the existing SEMs is implemented with full resistance reduction measures, it is practically difficult to further reduce the resistance.

The multiplication rate (gain) of about 105to 106is needed for converting an electron with only the elementary charge of monovalence into an electric signal by the detector and for discriminating the electric signal from noise. The gain is essential to achievement of high S/N. If the number of secondary electrons incident into the detector within a fixed time largely increases in order to meet the need for achievement of a higher throughput, while the scanning distance becomes longer with narrow focusing of the electron beam irradiating the sample, S/N will become lower due to the phenomenon of exceeding the upper limit of linearity of MCP. Namely, because of the upper limit of linearity determined by the MCP resistance, the maximum amount of incident electrons and the gain have a trade-off relation (as expressed by Expression (1) below).
(Upper limit of linearity of MCP)=(maximum amount of incident electrons)×(gain)   (1)

The present invention provides the SEM capable of performing the surface analysis of the sample at a high throughput, based on the Inventors' research as described above, and, particularly, is characterized by the configuration of the detector. An embodiment of the SEM of the present invention will be described below.

FIG. 1is a drawing showing a schematic configuration of the scanning electron microscope (SEM)1according to the embodiment of the invention. The SEM1has an electron gun2, an anode3, an alignment coil4, a condenser lens5, a deflection scanning coil6, a reflected electron detector7, a secondary electron detector8, an objective9, a laser interferometer10, a stage11, and a motor12, in a vacuum housing. Among these, the alignment coil4, condenser lens5, deflection scanning coil6, and objective9constitute an irradiation unit for scanning a sample20by irradiating the sample20with an electron beam and moving the position of irradiation with the electron beam on the sample20.

The electron gun2generates the electron beam. The election gun2is comprised, for example, of a tungsten wire. The tip of the tungsten wire is so shaped as to be sharp and have a predetermined face, and the electron gun2processed in this shape generates the electron beam with application of a high voltage thereto relative to the anode3. With application of the high voltage to the electron gun2, the anode3accelerates electrons emitted from the electron gun2. The alignment coil4deflects the electron beam emitted from the electron gun2so as to implement alignment. The condenser lens5focuses the electron beam emitted from the electron gun2. The deflection scanning coil6implements X-directional and Y-directional scanning with the electron beam impinging upon the sample20.

The reflected electron detector7detects reflected electrons generated in the sample20upon irradiation of the sample20with the electron beam. The reflected electron detector7has a circular disk shape with a hole in its center for letting the electron beam pass, which has, for example, four sectors obtained by quartering the entire circumference, so that it can obtain a reflected electron image with density differences depending upon the numbers of electrons based on the sum of signals from the four quartered sectors or so that it can provide highlighted display of unevenness of shape based on difference between two right and left signals. The secondary electron detector8performs collection, detection, and amplification of secondary electrons generated in the sample20upon irradiation of the sample20with the electron beam. The secondary electron detector8has a circular disk shape with a hole in its center for letting the electron beam pass. A secondary electron image can be obtained based on a signal detected by the secondary electron detector8.

The objective9narrowly focuses the electron beam on the surface of the sample20. The laser interferometer10measures the position of the stage11by making a laser beam outputted toward a reflector attached to the stage11interfere with a laser beam reflected by the reflector. The stage11is a device on which the sample20is mounted and which accurately moves the sample20to an arbitrary coordinate position, and is driven by the motor12. The motor12is a power supply for driving the stage11in X- and Y-directions, and examples thereof suitably applicable herein include a servo motor, an ultrasonic motor, and so on.

FIGS. 2A and 3Aare drawings showing a configuration of a detector100A applicable to the SEM shown inFIG. 1. This detector100A is applicable to the secondary electron detector inFIG. 1. The detector100A may be applicable to the reflected electron detector8. The detector100A includes, as a first structure, a laminate consisting of MCP111and MCP112(hereinafter referred to as “MCP laminate”), an anode120A, a dynode130A, and a bleeder circuit200A connected to an external power supply300A. The bleeder circuit200A applies predetermined voltages to respective electrodes, for forming a potential gradient as in the example shown inFIG. 3B.

In the detector100A having this first structure, each of the MCPs111,112is a secondary electron multiplier having the structure with a plurality of micro-channels arranged two-dimensionally and independently of each other. Each channel has the inner diameter of about 10 μm and is inclined at about 10° relative to a normal direction (coincident with a direction of incidence of electrons) to an input face of the MCP laminate (hereinafter referred to as “MCP input face”). It is noted, however, that the inclination direction of each channel in the MCP111is different from that in the MCP112. A lead wire114extending from the bleeder circuit200A is connected through an in electrode (hereinafter referred to as “MCP-IN electrode”)113to the MCP input face. Similarly, a lead wire116extending from the bleeder circuit200A is connected through an out electrode (hereinafter referred to as “MCP-OUT electrode”)115to an output face of the MCP laminate (hereinafter referred to as “MCP output face”). Namely, the bleeder circuit200A applies the predetermined voltages to the respective MCP-IN electrode113and MCP-OUT electrode115through the lead wires114,116, whereby the MCP input face and the MCP output face are set at the respective predetermined potentials. The output face is set at the potential higher than that of the input face, whereby the MCP laminate multiplies secondary electrons generated in accordance with arrival of secondary electrons at the input face and outputs the multiplied electrons from the output face.

The dynode130A is disposed on the side where the MCP output face lies (or on the opposite side to the MCP input face with respect to the MCP output face) and is configured to multiply the secondary electrons outputted from the MCP output face. The bleeder circuit200A is connected through a lead wire131A to the dynode130A and, the bleeder circuit200A applies the predetermined voltage to the dynode130A whereby the dynode130A is set at the potential higher than that of the MCP output face. The dynode130A is a metal flat plate (e.g., a SUS flat plate) arranged in parallel to the MCP output face. The dynode130A is preferably configured so that a surface of the metal flat plate (the face facing the MCP output face) is coated with a high-δ film (a film with a high secondary electron emission efficiency). The high-δ film is, for example, an alkali metal film, which is preferably an MgF2film.

The anode120A is disposed in parallel to the MCP output face, in a space from the dynode130A to an intermediate position between the MCP output face and the dynode130A. The anode120A may be located at the intermediate position between the MCP output face and the dynode130A. The anode120A has an aperture for letting the secondary electrons outputted from the MCP output face, pass toward the dynode130A. The anode120A is connected to a lead wire121A and the electric pulse signal outputted from the anode120A is amplified by an amplifier (Amp)250. The condenser251is disposed between the anode120A and the amplifier250, and the condenser251has a function keeping a signal output level at the GND level by insulating the output thereof. The anode120A is set at the potential higher than that of the dynode130A and is configured to collect the secondary electrons multiplied by the dynode130A. An aperture rate of the anode120A is preferably not more than 90%. Furthermore, the anode120A is preferably configured in a mesh shape with a plurality of apertures arranged two-dimensionally.

A through hole is provided in each central region of the MCP laminate (FIG. 2B), the anode120A (FIG. 2C), and the dynode130. A (FIG. 2D), and then these through holes are inserted with the tube member160. The electron beam to be incident to the sample20passes through an inner space of the tube member160.

The anode120A is sandwiched between a ceramic plate141and a ceramic plate142. The dynode130A is sandwiched between the ceramic plate142and a ceramic plate143. Each of the MCP-IN electrode113, MCP-OUT electrode115, and ceramic plates141-143has an annular shape. A relative positional relationship among the MCP-IN electrode113, MCP-OUT electrode115, and ceramic plates141-143is fixed with screws151,152, thereby assembling the detector100A having the first structure.

In this detector100A, the anode120A and dynode130A are arranged in order along the direction from the MCP input face to the MCP output face. The bleeder circuit200A applies the predetermined voltages to these respective electrodes through the lead wires114(the GND potential in the example inFIGS. 2A, 3A, and 3B),116,121A, and131A so that the potential of the dynode130A is higher than the potential of the MCP output face and so that the potential of the anode120A is higher than the potential of the dynode130A. When an electron arrives at the MCP input face, secondary electrons generated in response to the arrival of the electron are multiplied in the MCPs111,112. A large number of secondary electrons thus multiplied are outputted from the MCP output face. Most of the large number of secondary electrons outputted from the MCP output face pass through the aperture of the anode120A to collide with the dynode130A and this collision causes the dynode130A to generate a larger number of secondary electrons. The anode120A collects the larger number of secondary electrons generated by the dynode130A. Namely, when electrons arrive at the MCP input face, the anode120A outputs the electric pulse signal having a crest value depending on the number of electrons.

In an example of the potential gradient shown inFIG. 3B, the potential V1 of the MCP input face (MCP-IN electrode113) is set at 0V (GND potential), the potential V2 of the MCP output face (MCP-OUT electrode115) is set at +2000V, the potential V3 of the anode120A is set at +2500V, and the potential V4 of the dynode130A is set at a positive potential in the range (V4-setting range) of from V2 to V3. Regarding the potential gradient from MCP-IN electrode113to the anode120A, as described in the following experimental example, the potential V3 of the anode120A may be set at 0V (GND potential). In this case, as an example, the potential V1 of the MCP-IN electrode113is set at −2500V, the potential V2 of the MCP-OUT electrode115is set at −500V, the potential V3 of the anode120A is set at 0V (GND potential), and the potential V4 of the dynode130A is set at a negative potential in the range (V4-setting range) of from V2 to V3.

FIG. 4is a graph showing a gain characteristic of the detector100A. The horizontal axis represents the gain and the vertical axis the pulse count of electrons outputted from the MCP output face. In both of the detector100A with the first structure and a comparative example, the distance between the MCP output face and the anode120A was 1 mm and the distance between the anode120A and the dynode130A 1 mm. The dynode130A was a SUS plate coated with an MgF2film. The potential V1 of the MCP input face was −2500V, the potential V2 of the MCP output face −500V, and the potential V3 of the anode120A 0V (GND potential). In the comparative example, the potential V4 of the dynode130A was set at 0V (GND potential) and the anode120A and dynode130A were bundled so as to detect all the secondary electrons outputted from the MCP. In the detector100A applied to the present embodiment, secondary electrons were detected by the anode120A while the potential V4 of the dynode130A was set at −250V.

As can be seen fromFIG. 4, the gain of the detector100A was approximately 6.3 times the gain of the comparative example. A sub-peak is recognized at the position of the gain peak of the comparative example in the gain characteristic of the detector100A, and this indicates that some of the large number of secondary electrons outputted from the MCP output face are directly captured by the anode120A without reaching the dynode130A. In the description hereinbelow, a ratio of the gain of the detector100A (where the potential V3 of the anode120A is set higher than the potential V4 of the dynode130A) to the gain of the comparative example (where the anode120A and dynode130A are bundled so as to set the anode120A and dynode130A at the same potential) will be referred to as “relative gain.”

FIG. 5is a graph showing linearity characteristics of the detector100A. The horizontal axis represents the output current value (A) from the anode120A and the vertical axis the normalized gain. The normalized gain is defined as100for the gain at small output current values. InFIG. 5, mark “●” indicates the linearity characteristic with the potential V4 of the dynode130A set at the same potential as the potential V3 of the anode120A, mark “▪” the linearity characteristic with the potential V4 of the dynode130A set at −100V with respect to the potential V3 of the anode120A, mark “♦” the linearity characteristic with the potential V4 of the dynode130A set at −200V with respect to the potential V3 of the anode120A, mark “▴” the linearity characteristic with the potential V4 of the dynode130A set at −300V with respect to the potential V3 of the anode120A, combined mark of “*” and “−” the linearity characteristic with the potential V4 of the dynode130A set at −400V with respect to the potential V3 of the anode120A, and mark “×” the linearity characteristic with the potential V4 of the dynode130A set at −500V with respect to the potential V3 of the anode120A. In both of the detector100A and the comparative example used in this measurement, the distance between the MCP output face and the anode120A was 1 mm and the distance between the anode120A and the dynode130A 1 mm. The dynode130A was a SUS plate coated with an MgF2film. The potential V1 of the MCP input face was set at −2500V the potential V2 of the MCP output face at −500V, and the potential V3 of the anode120A at 0V (GNU potential). In the comparative example, the potential V4 of the dynode130A was set at 0V and the dynode130A and anode120A were bundled. As can be seen fromFIG. 5, the DC linearity of the detector100A with the potential V4 of the dynode130A set at −200V with respect to the potential V3 of the anode120A, was expanded to approximately seven times that of the comparative example.

It is understood fromFIGS. 4 and 5that in the detector100A applied to the present embodiment the linearity is also expanded by the degree of multiplication of the gain with respect to the comparative example.

FIGS. 6A to 6Care graphs showing relations between dynode potential V4 and relative gain of the detector100A, which were measured with variation in aperture rate of the anode120A.FIG. 6Ashows the relation obtained with the aperture rate of the anode120A of 81%.FIG. 6Bshows the relation obtained with the aperture rate of the anode120A of 90%.FIG. 6Cshows the relation obtained with the aperture rate of the anode120A of 96%. In the detector100A used in this measurement, the dynode130A was a SUS plate not coated with the high-δ film. The potential V1 of the MCP input face was −2500V, the potential V2 of the MCP output face −500V, and the potential V3 of the anode120A 0V (GND potential). The varying potential range of the dynode130A was from −50V to −500V. Each ofFIGS. 6A to 6Cshows measured values obtained in each of configuration wherein the distance between the MCP output face and dynode130A is 2.0 mm and wherein the ratio d1/d2of the distance d1between the MCP output face and the anode120A to the distance d2between the anode120A and the dynode130A is 0.5 mm/1.5 mm, 1.0 mm/1.0, mm, or 1.5 mm/0.5 mm.

As can be seen from theseFIGS. 6A to 6C, the relative gain is greater in the case of the distance d2being 1.0 mm than in the case of the distance d2being 1.5 mm between the anode120A and the dynode130A and the relative gain is much greater in the case of the distance d2being 0.5 mm. Therefore, the anode120A is preferably located in the space from the dynode130A to the intermediate position between the MCP output face and the dynode130A (or the anode120A may be located at the intermediate position between the MCP output face and the dynode130A) because the relative gain can be kept large. The difference in relative gain becomes more prominent as the potential difference between the anode120A and the dynode130A becomes smaller or as the aperture rate of the anode120A becomes smaller. Therefore, the aperture rate of the anode120A is preferably not more than 90%.

Next, the detector100B with the second structure, which is applicable to the SEM1inFIG. 1, will be described with reference toFIGS. 7A, 8A-8B, and 9.FIGS. 7A and 8Aare drawings showing the configuration of the detector100B applicable to the secondary electron detector8(or reflected electron detector7) of the SEM1inFIG. 1. This detector100B includes, as the second structure, the MCP laminate consisting of the MCP111and MCP112, an anode120B, an electrode130B, and a bleeder circuit200B connected to an external power supply300B. The bleeder circuit200B applies predetermined voltages to the respective electrodes, for forming a potential gradient as in the example shown inFIG. 8B.

In the detector100B having this second structure, each of the MCPs111,112is a secondary electron multiplier having the structure with a plurality of micro-channels arranged two-dimensionally and independently of each other. Each channel has the inner diameter of about 10 μm and is inclined at about 10° relative to the normal direction to the MCP input face. It is noted, however, that the inclination direction of each channel in the MCP111is different from that in the MCP112. The lead wire114extending from the bleeder circuit200B is connected through the MCP-IN electrode113to the MCP input face. Similarly, the lead wire116extending from the bleeder circuit200B is connected through the MCP-OUT electrode115to the MCP output face. Namely, the bleeder circuit200B applies the predetermined voltages to the respective MCP-IN electrode113and MCP-OUT electrode115through the lead wires114,116, whereby the MCP input face and the MCP output face are set at the respective predetermined potentials. The output face is set at the potential higher than that of the input face, whereby the MCP laminate multiplies secondary electrons generated in accordance with arrival of electrons at the input face and outputs the multiplied secondary electrons from the output face.

The anode120B is disposed on the side where the MCP output face lies (or on the opposite side to the MCP input face with respect to the MCP output face). The bleeder circuit200B is connected through a lead wire121B to the anode120B and, the bleeder circuit200B applies the predetermined voltage to the anode120B whereby the anode120B is set at the potential higher than that of the MCP output face. The anode120B is a metal flat plate (e.g., a SUS flat plate) arranged in parallel to the MCP output face and is set at the potential higher than that of the MCP output face so as to collect the secondary electrons outputted from the MCP output face. The electric pulse signal outputted from the anode120B is amplified by the amplifier (Amp)250. The condenser251is disposed between the anode120B and the amplifier250, and the condenser251has a function keeping a signal output level at the GND level by insulating the output thereof.

The electrode130B is disposed in parallel to the MCP output face, in a space from the anode120B to an intermediate position between the MCP output face and the anode120B. The electrode130B may be located at the intermediate position between the MCP output face and the anode120B. The electrode130B has an aperture for letting the secondary electrons outputted from the MCP output face, pass toward the anode120B. The electrode130B is connected to a lead wire131B and the electrode130B is set at the potential higher than that of the anode120B. An aperture rate of the electrode130B is preferably not more than 90%. Furthermore, the electrode130B is preferably configured in a mesh shape with a plurality of apertures arranged two-dimensionally.

A through hole is provided in each central region of the MCP laminate (FIG. 7B), the electrode130B (FIG. 7C), and the anode120B (FIG. 7D), and then these through holes are inserted with the tube member160. The electron beam to be incident to the sample20passes through an inner space of the tube member160.

The electrode130B is sandwiched between the ceramic plate141and the ceramic plate142. The anode120B is sandwiched between the ceramic plate142and the ceramic plate143. Each of the MCP-IN electrode113, MCP-OUT electrode115, and ceramic plates141-143has an annular shape. The relative positional relationship among the MCP-IN electrode113, MCP-OUT electrode115, and ceramic plates141-143is fixed with the screws151,152, thereby assembling the detector100B having the second structure.

In this detector100B, the electrode130B and anode120B are arranged in order along the direction from the MCP input face to the MCP output face. The bleeder circuit200B applies the predetermined voltages to these respective electrodes through the lead wires114(the GND potential in the example inFIGS. 7A, 8A, and 8B),116,121B, and131B (a positive potential in the example inFIGS. 7A, 8A, and 8B) so that the potential of the anode120B is higher than the potential of the MCP output face and so that the potential of the electrode130B is higher than the potential of the anode120B. When an electron arrives at the MCP input face, secondary electrons generated in response to the arrival of the electron are multiplied in the MCPs111,112. A large number of secondary electrons thus multiplied are outputted from the MCP output face and accelerated toward the anode120B by the electrode130B. As a result, most of the large number of secondary electrons outputted from the MCP output face pass through the aperture of the electrode130B to be collected by the anode120B. Namely, when electrons arrive at the MCP input face, the anode120B outputs the electric pulse signal having a crest value depending on the number of electrons.

In an example of the potential gradient shown inFIG. 8B, the potential V1 of the MCP input face (MCP-IN electrode113) is set at 0V (GND potential), the potential V2 of the MCP output face (MCP-OUT electrode115) is set at +2000V, the potential V3 of the anode120B is set at +2100V, and the potential V4 of the electrode130B is set at a positive potential (for, example +2500V) in the range (V4-setting range) exceeding the V2. Regarding the potential gradient from MCP-IN electrode113to the anode120B, as described in the following experimental example, the potential V3 of the anode120B may be set at 0V (GND potential). In this case, as an example, the potential V1 of the MCP-IN electrode113is set at −2300V, the potential V2 of the MCP-OUT electrode115is set at −500V, the potential V3 of the anode120B is set at 0V (GND potential), and the potential V4 of the dynode130B is set at a positive potential (for example, +500V) in the range (V4-setting range) exceeding the V3.

FIG. 9is a graph showing linearity characteristics of the detector100B. The horizontal axis represents the output current value (A) from the anode120B and the vertical axis the normalized gain. The normalized gain is defined as 100 for the gain at small output current values. InFIG. 9, mark “♦” indicates the linearity characteristic with the potential V4 of the dynode130B set at the same potential as the potential V3 of the anode120B, mark “▪” the linearity characteristic with the potential V4 of the electrode130B set at +100V with respect to the potential V3 of the anode120B, mark “▴” the linearity characteristic with the potential V4 of the electrode130B set at +200V with respect to the potential V3 of the anode120B, mark “×” the linearity characteristic with the potential V4 of the electrode130B set at +300V with respect to the potential V3 of the anode120B, mark “*” the linearity characteristic with the potential V4 of the electrode130B set at +400V with respect to the potential V3 of the anode120B. In the detector100B used in this measurement, the distance between the MCP output face and the electrode130B was 1 mm and the distance between the electrode130B and the anode120B 1 mm. The anode120B was a SUS plate. The potential V1 of the MCP input face was set at −2300V, the potential V2 of the MCP output face at −1500V, and the potential V3 of the anode120B at 0V (GND potential). As can be seen fromFIG. 9, the detector100B with the second structure also expanded the DC linearity by keeping the potential difference between the electrode130B and anode120B, for example, not less than 200V. It is understood that the linearity is also expanded by the degree of multiplication of the gain in the detector100B applied to the present embodiment, compared to the comparative example ofFIG. 5.

The detector100A or the detector100B with the structure as described above is applied to the detector8(or the detector7) of the SEM1of the first embodiment. Therefore, even with increase in amount of incident electrons to the detector100A or the detector100B, the gain of the entire detector can be kept large while restraining increase in gain of the MCPs111,112. Therefore, the SEM1according to the present invention can perform the surface analysis of the sample at a high throughput. Since each of the detector100A and detector100B can keep the gain of the MCPs111,112low, the voltage applied between the input face and output face of the MCP laminate can be set low, which improves life characteristics. The detector100A has a configuration in which the anode120A is inserted between the MCP laminate and the dynode130A, and the detector100B has a configuration in which the electrode130B is inserted between the MCP laminate and the anode120B. Therefore, these can suppress increase in scale, compared to the conventional configurations.

From the above description of the present invention, it will be obvious that the present invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the present invention, and all improvements as would be obvious to those skilled in the art are intended for inclusion within the scope of the following claims.