JP2006286207A - electronic microscope - Google Patents

Abstract

【Task】
An object of this invention is to implement | achieve the coexistence of the detection efficiency of the electron discharge | released from a sample, and the energy discrimination capability of an energy filter.
[Solution]
In the present invention, in order to achieve the above object, the energy filter is configured to include at least two cylinders, and the two cylinders are electrodes extending in the optical axis direction of the electron beam, and at least of the two electrodes. A part was configured to be positioned at the same height as viewed from the sample. According to such a configuration, it is possible to improve both the detection efficiency of electrons emitted from the sample and the energy discrimination capability of the energy filter.
[Selection] Figure 1

Description

  The present invention relates to an electron microscope, and more particularly, to a scanning electron microscope provided with an energy filter for energy discrimination of electrons emitted from a sample.

  A scanning electron microscope (abbreviated as SEM) accelerates electrons emitted from a field-emission electron source to form a thin electron beam (primary electron beam) using an electrostatic or magnetic lens, and this primary electron The beam is scanned two-dimensionally on the sample to be observed, and secondary signal electrons such as secondary electrons or reflected electrons generated secondary from the sample by the primary electron beam irradiation are detected, and the detected signal intensity is changed to the primary electron beam. A two-dimensional scanned image is obtained by using the luminance modulation input of a monitor that is scanned in synchronization with the scanning.

  In recent years, as the miniaturization of the semiconductor industry has progressed, SEM is used in the process of manufacturing semiconductor devices or inspection after completion of the process (for example, dimensional measurement using an electron beam or inspection of electrical operation) instead of an optical microscope. Became. In a sample (wafer) in the semiconductor industry in which an insulator is used, the insulator wafer is charged, and the wafer surface potential does not match the potential of the wafer holder. The variation of the wafer surface potential causes a variation in incident energy of the primary electron beam and a magnification error, which causes a dimensional error.

  As a method for detecting the surface charge of the wafer, an energy filter that conventionally discriminates electrons emitted from a sample has been used. A typical example is disclosed in Patent Document 1.

WO99 / 46798

  Patent Document 1 discloses various forms of energy filters. However, there are the following problems. For example, Patent Document 1 discloses that energy filtering of electrons emitted from a sample is performed by arranging a plurality of mesh filters, but the mesh electrode has a physical size, so There is a problem that the detection efficiency is lowered due to collision with the mesh.

  In addition, Patent Document 1 discloses that energy filtering is performed using an energy filter composed of two electrodes. However, in order to perform energy filtering because an electric field enters from the outside of the two electrodes. There is a problem that the negative potential region is limited and the energy resolution is lowered.

  An object of this invention is to implement | achieve the coexistence of the detection efficiency of the electron discharge | released from a sample, and the energy discrimination capability of an energy filter.

  In the present invention, in order to achieve the above object, the energy filter is configured to include at least two cylinders, and the two cylinders are electrodes extending in the optical axis direction of the electron beam, and at least of the two electrodes. A part was configured to be positioned at the same height as viewed from the sample.

  According to the above configuration, since it is possible to prevent the intrusion of the electric field from the outside without restricting the electron passage opening of the energy filter, the detection efficiency of the electrons emitted from the sample is improved, and the energy filter It is possible to improve both the energy discrimination capability.

  FIG. 1 shows an example of an SEM according to this example. The primary electron beam 1 is generated by applying an extraction voltage between the field emission cathode 11 and the extraction electrode 12. Further, the primary electron beam 1 is accelerated by a voltage applied to the acceleration electrode 13.

  The primary electron beam 1 that has passed through the diaphragm 15 is converged by the condenser lens 14 and subjected to scanning deflection by the upper scanning deflector 21 and the lower scanning deflector 22. The deflection intensities of the upper scanning deflector 21 and the lower scanning deflector 22 are adjusted so as to scan the sample 23 two-dimensionally with the lens center of the objective lens 17 as a fulcrum. The adjustment knob 16 moves the diaphragm 15.

  The deflected primary electron beam 1 is further accelerated by a subsequent acceleration voltage 19 by an acceleration cylinder 18 provided in the path of the objective lens 17. The post-accelerated primary electron beam 1 is narrowed down on the sample 23 by the lens action of the objective lens 17.

  A retarding voltage 25 is applied to the sample stage 24 on which the sample 23 is arranged, and the primary electron beam 1 that reaches the sample 23 is decelerated as compared to when it passes through the objective lens 17.

When the primary electron beam 1 irradiates the sample 23, secondary electrons 2 to 3 are generated. The secondary signals considered here are secondary electrons and reflected electrons. The electric field created between the objective lens 17 and the sample 23 acts as an accelerating electric field on the generated secondary signal, so that it is attracted into the path of the objective lens 17 and acts on the lens by the magnetic field of the objective lens 17. Ascend while receiving. The secondary electrons 2 to 3 that have passed through the objective lens 17 pass through the scanning deflectors 21 and 22 and enter the energy filter 40.

The energy filter 40 includes a thin-walled cylinder 42 concentrically inside a cylinder 41, and a filter voltage in which a retarding voltage 25 and a filter power supply 45 are superimposed is applied to the cylinders 41 and 42.

  By arranging the cylinder 42, the influence of the ground voltage penetrating from the upper and lower openings of the cylinder 41 is reduced, and a uniform filter voltage can be formed. For this reason, the transmission and non-transmission of the two secondary electrons 2 and 3 having the same energy do not depend on the incident position on the filter, and the energy resolution is improved.

  The electrons that have passed through the cylinders 41 and 42 collide with the secondary electron conversion electrode 29 and generate secondary electrons 4. The secondary electrons 4 are deflected toward the scintillator 5 by a deflection electric field formed by the deflection electrode 26. When the secondary electrons 4 collide, the scintillator 5 generates light and is guided to the light guide 6 and the photomultiplier tube 7 to be converted into an electric signal and amplified.

  The detector composed of the scintillator 5 and the like is arranged on the electron source side from the cylinders 41 and 42. The deflection coil 28 forms a magnetic field in a direction orthogonal to the deflection electric field formed by the deflection electrode 26 and the optical axis of the primary electron beam, and performs deflection to cancel the deflection of the primary electron beam due to the deflection electric field.

  Below, the specific structure of the energy filter 40 is demonstrated.

  FIG. 2 schematically shows the configuration and problems of an energy filter composed of a single cylinder. The secondary electrons 2 and 3 generated in the sample 23 to which the retarding voltage 25 is applied pass through the cylinder 20 of the scanning deflector and enter the cylinder 41 of the energy filter. The cylinder 41 is applied with a filter voltage in which the retarding voltage 25 and the filter power supply 45 are superimposed.

  Although it is desirable that the inside of the cylinder 41 has a uniform potential distribution due to the filter voltage, in reality, the ground voltage of the surrounding electrodes (the cylinder 20 and the mesh 30) penetrates from the upper and lower openings of the cylinder, and the uniform potential distribution. Cannot be formed. For this reason, the transmission and non-transmission of the two secondary electrons 2 and 3 having the same energy depend on the incident position on the filter, which causes the energy resolution to deteriorate.

  In order to improve the energy resolution of the energy filter composed of a single cylinder, it is conceivable to reduce the diameter of the filter cylinder 41 or to narrow a part of the diameter. If the diameter of the filter cylinder is reduced, the penetration of the electric field from the outside can be suppressed, but the trajectory of secondary electrons and reflected electrons far away from the optical axis will be limited, so it was discriminated by the energy filter. Not only does the S / N of an image made of electrons (for example, reflected electrons) decrease, but the S / N of the image also decreases under the condition that no energy filter is used.

  As an example of improving the energy resolution without reducing the diameter of the filter cylinder 41, a plurality of meshes 44 may be stretched so as to cover the opening of the cylinder 41 as shown in FIG. Even when this method is used, the transmittance of secondary electrons is reduced by the frame portion of the mesh 44, and the S / N ratio of the image is lowered even under the condition where the energy filter is not used.

  From the above viewpoint, in the example shown below, it is proposed that a plurality of cylinders are stacked in order to form a uniform potential distribution in the filter cylinder without reducing the diameter of the filter cylinder.

  FIG. 4 shows a characteristic configuration of this example. The secondary electrons 2 and 3 generated in the sample 23 to which the retarding voltage 25 is applied pass through the cylinder 20 of the scanning deflector and enter the cylinder 41 of the energy filter. A thin-walled cylinder 42 is disposed concentrically inside the cylinder 41. The cylinders 41 and 42 are applied with a filter voltage in which the retarding voltage 25 and the filter power supply 45 are superimposed. By arranging the cylinder 42, the influence of the ground voltage penetrating from the upper and lower openings of the cylinder 41 is reduced, and a uniform filter voltage can be formed. For this reason, the transmission and non-transmission of the two secondary electrons 2 and 3 having the same energy do not depend on the incident position on the filter, and the energy resolution is improved.

  The cylinders 41 and 42 are electrodes extending in the irradiation direction (optical axis direction) of the primary electron beam 1, and at least a part of the two electrodes is positioned at the same height when viewed from the sample 23. FIG. 4 is a diagram for explaining an example in which the cylinders 41 and 42 have exactly the same length in the optical axis direction and are positioned at the same height, but a uniform potential distribution is present between the cylinders 41 and 42. The cylinders 41 and 42 do not necessarily have the same length as long as they are arranged to have a predetermined length in the optical axis direction of the primary electron beam. Further, if at least a part of them is arranged at the same height with respect to the sample and a uniform potential distribution of a predetermined length is formed between them, it is not necessary to position them completely at the same height.

  However, if the lengths of the cylinders 41 and 42 are different or there is a difference in the arrangement height, the trajectory of electrons emitted from the sample may be changed, which may reduce the electron detection efficiency. It is desirable that the cylinders 41 and 42 have the same length in the optical axis direction and are positioned at the same height.

  The cylinders 41 and 42 are formed so that the primary electron beam optical axis passes through the center of the cylinder, and the wall surfaces thereof are formed parallel to the primary electron beam optical axis. The primary electron beam optical axis in this example is a passing trajectory through which a primary electron beam not subjected to deflection passes.

  Further, FIG. 4 is a view showing an example in which the cylinders 41 and 42 are formed so as to completely surround the primary electron beam optical axis in the 360 ° direction (configured so that there is no hole in the side of the cylinder). However, the present invention is not limited to this. For example, the wall surface of the cylinder may be configured in a mesh shape, or a plurality of plate-like bodies or ring-like bodies are arranged in a cylindrical shape to form a substantial cylinder. You may make it do.

  However, it should be noted here that it is necessary to form such that other electric fields from the outside of the cylinder do not greatly affect the uniform potential distribution formed in the cylinder. For example, when there is a large opening on the side of the cylinder 41, an external electric field may ooze out from the cylinder. As a result, the energy resolution of the region where the external electric field oozes out decreases.

  When such an opening is formed only in a certain direction of the cylinder, there is a difference in energy resolution between the vicinity of the opening and other regions. That is, there arises a problem that the energy discrimination ability varies depending on the electron emission direction from the sample.

  In order to solve the above problems, it is desirable to configure the cylinders 41 and 42 to be axially symmetric with respect to the primary electron beam optical axis.

  According to this example, an energy filter with high resolution can be created without reducing the diameter of the filter cylinder. As a result, reflected electrons and secondary electrons far away from the optical axis can be detected, and not only the S / N of the reflected electron image consisting of the reflected electrons discriminated by the energy filter is improved, but S An image with a high / N can be obtained.

  FIG. 5 is a more specific configuration example of the cylinders 41 and 42. The cylinder 42 is fixed to the cylinder 41 by a column 43. The support column 43 is formed of a thin member so as to reduce the area in the direction perpendicular to the optical axis of the primary electron beam as much as possible. With such a configuration, it is possible to arrange the cylinder 42 while suppressing a decrease in electron detection efficiency. Since the cylinder 42 is electrically connected to the cylinder 41, the same voltage as the voltage applied to the cylinder 41 is applied.

  In the above description, an example in which the cylinder is doubled has been described. However, the present invention is not limited to this, and it is also possible to have a triple cylinder or more. However, as the number of cylinders increases, the detection efficiency of electrons emitted from the sample decreases, so the appropriate shape and size of the energy filter is determined by the trade-off between the detection efficiency and the energy discrimination ability of the energy filter. It is desirable to do.

The whole block diagram of the scanning electron microscope which has an energy filter demonstrated in an Example. The figure explaining the problem of the energy filter which consists of a single cylinder. The figure explaining the problem at the time of improving energy resolution by stretching a plurality of meshes so that the opening of a filter cylinder may be covered. The figure explaining the effect at the time of using the energy filter demonstrated in an Example. The figure which shows the structural example of the cylinder which comprises an energy filter.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Primary electron beam, 2, 3 ... Secondary electron, 4 ... New secondary electron which secondary electron produced | generated, 5 ... Scintillator, 11 ... Field emission cathode, 12 ... Extraction electrode, 13 ... Acceleration electrode, 14 ... Condenser lens, 15 ... Aperture, 16 ... Adjustment knob, 17 ... Objective lens, 18 ... Accelerating cylinder, 19 ... Rolling acceleration voltage, 20 ... Cylinder of scanning deflector, 21 ... Upper scanning deflector, 22 ... Lower scanning deflector, 23 ... Sample, 24 ... Sample stage, 25 ... Retarding voltage, 26 ... Electrostatic deflection electrode, 27 ... Electrostatic deflection electrode (mesh), 28 ... Deflection coil, 29 ... Secondary electron conversion electrode, 30 ... Ground electrode ( Mesh), 40 ... energy filter, 41, 42 ... filter cylinder, 43 ... strut, 45 ... filter power supply, 49 ... equipotential lines when a voltage is applied to the filter.

Claims (4)

In an electron microscope equipped with an energy filter that discriminates electrons emitted from the sample by irradiating the sample with an electron beam,
The energy filter includes at least two cylinders, and the two cylinders are electrodes extending in an optical axis direction of the electron beam, and at least a part of the two electrodes is positioned at the same height as viewed from the sample. Electron microscope characterized by being made. In claim 1,
An electron microscope, wherein a negative voltage is applied to the two cylindrical electrodes. In claim 1,
2. The electron microscope according to claim 2, wherein the two cylindrical electrodes have a cylindrical shape that is axisymmetric with respect to an optical axis of the electron beam. In claim 1,
The two microscopes have the same length in the electron beam optical axis direction and are positioned at the same height when viewed from the sample.

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