JP2009205936A - Scanning electron microscope - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a scanning electron microscope having an objective lens capable of high resolution observation at a low acceleration voltage in which a high resolution observation can be made even at a high acceleration voltage. <P>SOLUTION: The scanning electron microscope in which an image of a test piece is obtained by a secondary signal generated by irradiating electron beams on the test piece is provided with a high voltage control power source to impress an acceleration voltage between a negative electrode and a second positive electrode of an electron source, an objective lens of lower magnetic pole opening type having a thin portion, and an objective lens control power source which changes excitation intensity by changing current flowing in a coil of the objective lens. When an acceleration voltage higher than a predetermined acceleration voltage is impressed, the objective lens control power source increases current flowing in the coil of the objective lens. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a scanning electron microscope that generates an image of a sample from a secondary signal obtained by irradiating the sample with an electron beam.

  A scanning electron microscope narrows down an electron beam generated from an electron source with a focusing lens and an objective lens and irradiates the sample while scanning the sample two-dimensionally, such as secondary electrons and reflected electrons generated from the sample. This is an apparatus for detecting a secondary signal and forming a scanning image of a sample from a signal obtained by synchronizing the detection signal and scanning of an electron beam.

  In recent years, in a scanning electron microscope, there is a high demand for high-resolution observation with an acceleration voltage as low as 2 kV or less. For this reason, the distance from the main surface of the objective lens is shortened by bringing the sample closer to the objective lens. . On the other hand, when elemental analysis of a sample using X-rays is performed, it is necessary to set a high acceleration voltage in order to emit X-rays from the sample. However, when the distance between the main surface of the objective lens and the sample is short, the magnetic field intensity necessary for focusing cannot be obtained due to saturation of the objective lens magnetic path, and the distance between the sample and the objective lens must be increased. .

  In the case of high resolution observation with a low acceleration voltage, it can be realized by moving the sample up and down so that the sample moves closer to the objective lens, and in the case of elemental analysis at a higher acceleration voltage, the sample is moved away from the objective lens. Is possible. However, since the waiting time for moving the sample up or down is long, and adjustment of the focus or the like is required after the sample is moved, it takes more time. When observing or inspecting defects on a semiconductor wafer on which a semiconductor device is formed as a sample, there is a demand for high throughput for the inspection apparatus in order to improve manufacturing yield. I do not get.

  On the other hand, as a technique for realizing high-speed focusing of a scanning electron microscope, for the purpose of achieving both high-resolution observation and X-ray analysis, the current flowing through the coil of the objective lens is turned off or reduced at a high acceleration voltage during X-ray analysis. A technique is known in which focusing is performed by setting excitation to OFF or weak excitation and providing a focusing lens arranged on the electron source side of the objective lens or an objective lens dedicated to a high acceleration voltage (for example, Patent Documents). 1).

  However, in the above method, the resolution is greatly reduced because the distance between the specimen and the main surface of the lens to be focused or the focal length is significantly different from that in the low acceleration voltage observation condition under the high acceleration voltage observation condition. End up. In recent years, not only X-ray analysis under high acceleration voltage conditions but also improvement in the resolution of the observation image of the sample has been demanded, and the above method cannot cope with this.

Japanese Patent Laid-Open No. 11-135052

  An object of the present invention is to provide a scanning electron microscope capable of high-resolution observation even at a high acceleration voltage in a scanning electron microscope provided with an objective lens capable of high-resolution observation at a low acceleration voltage.

  In order to solve the above problems, an embodiment of the present invention is a scanning electron microscope that obtains an image of a sample from a secondary signal generated by irradiating the sample with an electron beam, the cathode of the electron source, a second anode, A high-voltage control power source for applying an acceleration voltage between the objective lens and the objective lens control power source for changing the excitation intensity by changing the current flowing through the coil of the objective lens. When the acceleration voltage higher than the predetermined acceleration voltage is applied, the objective lens control power source is configured to increase the current flowing through the coil of the objective lens.

  According to an embodiment of the present invention, in a scanning electron microscope equipped with an objective lens capable of high-resolution observation at a low acceleration voltage, a scanning electron microscope capable of high-resolution observation even at a high acceleration voltage can be provided. it can.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a longitudinal sectional view showing a configuration of a main part of a scanning electron microscope, in which a vacuum column and a sample chamber for shielding from outside air are omitted. An electron source that generates an electron beam includes a cathode 1, a first anode 2, and a second anode 3. An extraction voltage is applied between the cathode 1 and the first anode 2 by a high voltage control power supply 30, and an electron beam 4 having a predetermined emission current is extracted from the cathode 1. Since an acceleration voltage is applied between the cathode 1 and the second anode 3 by the high voltage control power supply 30, the electron beam 4 emitted from the cathode 1 is accelerated and proceeds to the lens system at the subsequent stage.

  In the case of observation with a low acceleration voltage, an acceleration voltage lower than a predetermined voltage value is applied. In the case of observation with a high acceleration voltage or X-ray analysis, an acceleration voltage higher than a predetermined voltage value is applied. To do.

  An unnecessary energy region near the outer periphery of the electron beam 4 is limited by the diaphragm plate 5, and the electron beam 4 is focused by the focusing lens 6 whose excitation intensity is controlled by the focusing lens control power supply 31. Thereafter, the electron beam 4 is narrowed down with respect to the sample 8 by the objective lens 7 controlled by an objective lens control power supply 34 with a lower magnetic pole open type. On the other hand, the electron beam 4 is scanned by the scanning deflector control power source 33 so as to have a scanning width corresponding to the observation magnification, so that the electron beam 4 is finely focused on the surface of the sample 8. Deflected.

  The high voltage control power supply 30, the focusing lens control power supply 31, the scanning deflector control power supply 33, and the objective lens control power supply 34 are controlled by a control unit 50 including a microprocessor.

  Secondary electrons 11 generated from the sample 8 by the irradiation of the electron beam 4 are detected by the detector 9. The detection signal is amplified by the signal amplifier 32, subjected to signal processing synchronized with the scanning of the electron beam 4 by the control unit 50, and displayed on the image display device 35 as a sample image.

  As shown in FIGS. 1 and 2, the objective lens 7 is provided with a thin portion 12 in which a part of the magnetic pole is thinned in the vicinity of the lens gap 13, and the magnetic field of the objective lens 7 in observation at a high acceleration voltage is provided. In the distribution, magnetic field leakage is caused from the thin wall portion 12. When the current flowing through the coil of the objective lens 7 exceeds a predetermined value, the shape of the thin portion 12 is determined so that magnetic field leakage occurs. The thickness of the thin portion 12 is determined so that there is no or little magnetic field leakage from the thin portion 12 in the distribution of the magnetic field of the objective lens 7 in observation at a low acceleration voltage.

  FIG. 2 is an enlarged longitudinal sectional view of the objective lens shown in FIG. 1 and shows an on-axis magnetic field distribution 14 on the central axis of the objective lens 7. FIG. 2A shows an on-axis magnetic field distribution 14 at the central axis of the objective lens 7 at the time of observation of a low acceleration voltage. The main lens position 15a of the objective lens 7 is such that the objective lens 7 is a lower magnetic pole open type. Therefore, it exists at a position close to the sample 8. FIG. 2B shows an on-axis magnetic field distribution 14 in the central axis of the objective lens 7 when observing a high acceleration voltage, and the current of the coil of the objective lens 7 becomes a certain value or more. When the magnetic field leaks from the thin portion 12, the main surface position 15 b of the objective lens 7 exists at the position of the thin portion 12 in the direction away from the sample 8. If the main surface position 15b of the objective lens 7 is away from the sample 8, it is easy to focus the electron beam 4 on the sample 8, and the possibility of focus shift is reduced, so that the focusing time can be shortened.

  Further, since the main surface position 15b is formed in the thin portion 12 of the objective lens 7, the main surface position of the objective lens 7 is moved between the observation of the low acceleration voltage and the observation of the high acceleration voltage. The image resolution can be reduced and the image resolution can be reduced a little, so degradation of image resolution can be reduced.

  FIG. 3 is an enlarged longitudinal sectional view of the objective lens shown in FIG. 1, as in FIG. In FIG. 2, the thin portion 12 provided in the objective lens 7 has a notch shape having a vertical surface and a horizontal surface with respect to the upper surface of the sample 8, but as shown in FIG. Instead, the same effect can be obtained with a thin shape up to the magnetic pole tip of the objective lens 7.

The longitudinal cross-sectional view which shows the structure of the principal part of a scanning electron microscope. The longitudinal cross-sectional view which expanded the objective lens shown in FIG. The longitudinal cross-sectional view which expanded the objective lens shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Cathode 2 1st anode 3 2nd anode 4 Electron beam 5 Aperture plate 6 Focusing lens 7 Objective lens 8 Sample 9 Detector 10 Scanning deflector 11 Secondary electron 12 Thin part 13 Lens gap 14 On-axis magnetic field distribution 15a, 15b Main Surface position 30 High voltage control power supply 31 Focusing lens control power supply 32 Signal amplifier 33 Scanning deflector control power supply 34 Objective lens control power supply 35 Image display device 50 Control unit

Claims (4)

  A scanning electron microscope for obtaining an image of a sample from a secondary signal generated by irradiating the sample with an electron beam, a high voltage control power source for applying an acceleration voltage between a cathode and a second anode of the electron source, An objective lens having a lower magnetic pole opening type for narrowing the electron beam and having a thin wall portion, and an objective lens control power source for changing the excitation intensity by changing the current flowing through the coil of the objective lens, are predetermined. The scanning electron microscope according to claim 1, wherein when an acceleration voltage higher than the acceleration voltage is applied, the objective lens control power source increases a current flowing in a coil of the objective lens.   2. The scanning electron microscope according to claim 1, wherein when an acceleration voltage higher than the predetermined acceleration voltage is applied, a position of a main surface of the objective lens moves in a direction away from the sample. .   The main surface of the objective lens is moved to a thin portion provided in the objective lens when an acceleration voltage higher than the predetermined acceleration voltage is applied. Scanning electron microscope.   2. The scanning electron microscope according to claim 1, wherein the thin portion of the objective lens is shaped so that a magnetic field leaks when a current flowing through a coil of the objective lens becomes a predetermined value or more. .

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