Scanning Electron Microscope With Improved Means For Focusing

Abstract

A scanning electron microscope for observing a transmitted electron scanning image of a specimen incorporating a means for detecting electron beams transmitted in more than one direction separately and a means for displaying scanning images corresponding to said detected beams, thus facilitating the focusing adjustment of the condenser lens system incorporated in said scanning electron microscope.

Description

This invention relates in general to a scanning electron microscope and in particular to a means for monitoring the focusing condition of the electron beam irradiating the specimen under examination.

In a scanning electron microscope, in order to obtain a very small diameter beam necessary for high resolution scanning image observation, the electron beam generated by an electron gun is condensed by one or more condenser lenses. However, adjustment of the condenser lens excitation current for optimum beam focus is difficult for the average microscopist. A further inconvenience is the fact that the incorporated stigmator which requires adjustment in order to correct lens astigmatism, a phenomenon detrimental to obtaining optimum beam focus, is also most difficult to adjust precisely.

An advantage of this invention is to facilitate adjustment of the condenser lens excitation current for optimum beam focus. Another advantage of this invention is to facilitate adjustment of the stigmator.

These and other objects of the invention will become more readily apparent by reading the following description in conjunction with the accompanying drawings of which;

FIG. 1 is a schematic diagram showing one embodiment according to the invention;

FIG. 2 is a schematic drawing showing the images displayed on the screens of the cathode-ray tubes shown in FIG. 1;

FIG. 3 is a schematic drawing showing the electron beam path near a specimen for explaining the invention;

FIG. 4 is a schematic diagram showing another embodiment according to the invention;

FIG. 5 is a schematic drawing showing the image displayed on the screen of the cathode-ray tube shown in FIG. 4;

FIGS. 6, 7, 8 and 9 are schematic diagrams showing further embodiments according to the invention; and

FIG. 10 is a schematic drawing showing the image displayed on the screen of the cathode-ray tube shown in FIG. 9.

Referring to FIG. 1, a thin specimen 1 is arranged in an evacuated column 2 of a scanning electron microscope. The specimen 1 is irradiated by an electron beam EB1 generated by an electron gun 3. Excitation current supply sources 4 and 5 control the focal length of condenser lenses 6 and 7 to focus the electron beam EB1 on the specimen surface. An excitation current supply source 8 energizes and controls a stigmator 9 in order to correct lens astigmatism. Scanning coils 10X and 10Y scan the electron beam EB1 over the desired area of the specimen surface. A signal generator 11 energizes the scanning coils 10X and 10Y and controls the amount of scan. Beam detectors 12 and 13 detect the electrons transmitted through the specimen 1. The aperture plates 12a and 13a are arranged to pass only the transmitted electrons which pass along the optical axis and at a certain angle .alpha., respectively. The detected electron output signals, after being amplified by amplifiers 16 and 17, are applied to the control grids of cathode-ray tubes 14 and 15, respectively. The electron beams of said CRT's are scanned by scanning coils 14X, 14Y and 15X, 15Y and, since these scanning coils are energized and controlled by the same scanning signal generating source 11 as scanning coils 10X and 10Y, the scanning of the signals synchronizes with the scanning of the electron beam EB1. As a result, scanning images corresponding to electron beams EB2 and EB3 are displayed on the respective cathode-ray screens 14s and 15s.

If the electron beam EB1 is correctly focussed, the images on the respective screens will appear in identical positions. On the other hand, if EB1 is incorrectly focussed, there will be a positional difference D, as shown in FIG. 2, depending on how much the beam is out of focus. Thus, in the out of focus condition, it is comparatively easy to bring the beam into spot-on focus by adjusting this condenser lens excitation current until the two images are positioned identically.

FIG. 3 shows schematically the electron beam path in the vicinity of the specimen 1 under the condition that the electron beam is improperly focussed by a condenser lens (not shown) on a plane 18 above the specimen. The electrons which pass straight through the specimen along the optical axis 19 are detected by a detector A (not shown) and the electrons which pass through the specimen at angle .alpha..sub.1 with respect to said optical axis are detected by a second detector B (also not shown), said detectors corresponding to detectors 12 and 13 described in FIG. 1. Accordingly, detector A detects the electrons carrying information pertaining to a minute area 20 of the specimen while detector B detects the electrons carrying information pertaining to a second minute area 21 of said specimen. As the focussed beam is scanned, the transmitted beams move to positions as shown by the broken lines with the result that detector B now detects the electrons carrying information pertaining to said area 20. Thus, excepting the time lag, shown as a positional difference D in FIG. 2, the respective detector brightness modulation output signal is virtually the same. Angle .alpha..sub.1, in this case, is less than 10.sup.-.sup.4 rad which is not large enough to separate elastic and inelastic electrons in the specimen.

FIG. 4 shows another embodiment of this invention in which only one cathode-ray tube is used to display the two images. This is made possible by incorporating a switching circuit 23 between the output circuit of amplifiers 16 and 17 and the control grid of the single cathode-ray tube 22, said switching circuit being synchronized with the scanning signal generator 11. Thus, by alternately applying signals from detectors 12 and 13 to the control grid of the cathode-ray tube 22 via amplifiers 16 and 17, corresponding images are alternately displayed on the cathode-ray tube screen 22s once per scanning frame. Accordingly, if the condenser lens system is incorrectly adjusted, the image will vibrate by an amount D as shown in FIG. 5. Correct adjustment, hence spot-on focus, is obtained when the image ceases to vibrate.

FIG. 6 shows a variation of the embodiment described in FIG. 4. The means differ, however. In this case, instead of an electrical switching means, a mechanical vibrating means 24 is incorporated in conjunction with an aperture plate 25. Moreover, the two detectors 26 and 27 are arranged symmetrically with respect to the microscope optical axis. By vibrating the aperture plate 25, only one electron beam, either EB4 or EB5 is passed through the plate aperture, either 25a or 25b, at a given time. Accordingly, either a stationary or vibrating image will appear on the cathode-ray tube according to whether or not the condenser lens system is correctly or incorrectly adjusted.

The embodiment shown in FIG. 7 achieves the same object as the embodiment described in FIG. 6 using one wide window detector 28 instead of two smaller detectors, and an aperture plate 27 with single aperture 27c instead of two apertures.

FIG. 9 schematically illustrates the essential part of an embodiment designed to facilitate adjustment of the stigmator lens as the focusing lenses. A plurality of detectors and apertures; in this case six detectors 32, 33, 34, 35, 36 and 37 and six apertures 32a, 33a, 34a, 35a, 36a and 37a are symmetrically arranged about the optical axis 19 below the specimen 1. The output signals of the respective detectors, after being amplified by amplifiers 32c, 33c, 34c, 35c, 36c and 37c are applied to a switching circuit 38, the output of which is applied to the brightness control grid of the cathode-ray tube 22. The switching circuit 43 is synchronized with the scanning signal generator 11.

If lens astigmatism exists, the focal length of the lens will change as the azimuth direction with respect to the optical axis changes. Therefore, if the stigmator power supply 8 is correctly adjusted, D.sub.1, D.sub.2 and D.sub.3 are equal as shown in FIG. 10. Further, if the condenser lens system is also correctly adjusted, D.sub.1, D.sub.2 and D.sub.3 are equal and zero.

It will thus be appreciated that by being able to visually monitor the amount of image shift on the cathode-ray tube screen, stigmator and condenser lens adjustment for optimum conditions is greatly facilitated.

A further alternative on the embodiment shown in FIG. 7 is shown in FIG. 8. A deflecting coil 30 energized by a deflecting current source 31 synchronized with the scanning signal generator 11 replaces the previously described switching and vibrating means, etc. In this embodiment, by changing the intensity of the output current of the deflecting current source 31 in two or more steps in turn, the transmitted electron beams having different angles with respect to the optical axis are passed through the aperture 12a and detected by the detector 12. Accordingly, either a stationary or vibrating image will appear on the cathode-ray tube 22 according to whether or not the condenser lens system is correctly or incorrectly adjusted.

Claims

Having thus described the invention with the detail and particularity as required by the Patent Laws, what is desired protected by Letters Patent is set forth in the following claims:

1. In a scanning electron microscope incorporating an electron gun for generating an electron beam, an electron lens system for focusing the said beam on the surface of a thin specimen, a scanning means for scanning the beam over the specimen surface, the improvement comprising a detecting means for separately detecting the electrons transmitted through the specimen in different substantially discrete directions and a display means for simultaneously displaying the scanning images corresponding to the output signals of said detecting means, said display means enabling the observation of the positional difference of the said detectors when the electron beam is out of focus whereby the focus can be corrected by observing said display means.

2. The improvement in a scanning electron microscope set forth in claim 1 wherein said detecting means comprises two or more electron detectors arranged below the specimen such that the respective detectors detect the electrons transmitted through the specimen in different directions.

3. The improvement in a scanning electron microscope set forth in claim 1 wherein said detecting means comprises one electron detector arranged below the specimen, an aperture plate arranged above said detectors below the specimen and a means for vibrating said aperture plate.

4. The improvement in a scanning electron microscope set forth in claim 1 wherein said detecting means comprises two or more electron detectors arranged below the specimen, an aperture plate arranged over said two or more detectors below the specimen and a means for vibrating said aperture plate.

5. The improvement in a scanning electron microscope set forth in claim 1 wherein said detecting means comprises one electron detector arranged below the specimen and a deflecting means for alternatively deflecting the electron beam so as to vary the direction of the transmitted electron beam detected by said detector.

6. The improvement in a scanning electron microscope set forth in claim 1 wherein said display means comprises two or more cathode-ray tubes for displaying images corresponding to the respective directions of the transmitted electron beams.

7. The improvement in a scanning electron microscope set forth in claim 1 wherein said display means comprises one cathode-ray tube for alternately displaying images corresponding to the respective directions of the transmitted electron beams.