JP3156428B2 - Scanning electron microscope - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electron beam apparatus, and more particularly to a scanning electron microscope suitable for obtaining an extremely low magnification image and a similar apparatus.

[0002]

2. Description of the Related Art Conventionally, in a scanning electron microscope, in order to prevent image distortion during low-magnification observation, a two-stage deflection coil is arranged above an objective lens (on the side of an electron source), and a deflection fulcrum of a primary electron beam is used as an objective. The winding ratio (deflection ratio) of the two-stage deflection coil is set so as to be near the lens main surface. Generally, the image magnification in a scanning electron microscope is inversely proportional to the amount of deflection of a primary electron beam on a sample. Therefore, the lower the magnification of the primary electron beam on the sample, the lower the magnification of the observation. However, since the deflection fulcrum is set near the main surface of the objective lens, the amount of deflection is determined by the distance from the main surface of the objective lens to the sample, that is, the focal length of the objective lens. So, for example,
In order to display an image at an extremely low magnification of several tens of times or less, the distance (working distance) between the sample and the objective lens should be 30 mm to 4 mm.
It had to be used considerably longer, 0 mm. on the other hand,
In a so-called in-lens type scanning electron microscope in which a sample is placed in a lens magnetic field, a scanning microscopy (Vol.
1, No. 3, 1987, pp. 901-909), the focal length of the objective lens is very short, and a low magnification of 250 times or less cannot be obtained by the ordinary two-stage deflection method. For this reason, when displaying a low-magnification image, the excitation of the objective lens is turned off or weakly excited, and the deflection amount on the sample is increased by using a deflection coil in one stage. However, also in this case, there is a problem that the field of view is limited by the hole of the magnetic pole of the objective lens, and an extremely low magnification cannot be obtained.

[0003]

SUMMARY OF THE INVENTION It is an object of the present invention to eliminate the above-mentioned disadvantages of the prior art, and to provide a hole in a magnetic pole of an objective lens.
Easy observation of ultra-low magnification images without restricting the field of view
And to provide a current can scanning electron microscope.

[0004]

In order to achieve the above-mentioned object, the present invention changes the position of the deflection fulcrum between normal use at a high magnification and use at a very low magnification so as to reduce the very low magnification. To obtain the optimum condition, the deflection fulcrum is moved from the normal use position (near the main surface of the objective lens) to the electron source side. To change the deflection fulcrum, it is necessary to change the deflection ratio of the two-stage deflector. For this purpose, an auxiliary deflector is added to one of the two-stage deflectors, or the two-stage deflector is deflected. The drive circuits of the deflectors are made independent, and the deflection signal ratio of each deflector is made variable. By providing means for electrically changing the deflection fulcrum as described above, the deflection fulcrum suitable for normal high-resolution observation and the deflection fulcrum suitable for extremely low magnification can be electrically easily switched. On the other hand, since the position of the deflection fulcrum for obtaining the extremely low magnification is not near the main surface of the objective lens, image distortion occurs when the objective lens is in a normal excitation state (an excitation state in which a sample can be focused). for that reason,
Under ultra-low magnification conditions, the objective lens is set to a weakly excited state or an OFF state. At this time, a focusing lens arranged above the objective lens (on the side of the electron source) is used to focus the primary electron beam.

[0005]

According to the above-described characteristic configuration of the present invention, the following operation and effect can be obtained. That is, in ordinary high-resolution observation that does not require an extremely low magnification, the primary electron beam is deflected in the orbit shown in FIG. Since the deflection fulcrum at this time is set in the vicinity of the main surface of the objective lens, no distortion occurs in the image even when a normal focusing operation is performed by the objective lens. At the time of ultra-low magnification observation, when the excitation of the objective lens is turned off and the primary electron beam is deflected in the orbit shown in FIG. 3, an extremely low magnification lower than the lowest magnification in normal observation can be easily realized. The optimum position of the deflection fulcrum for extremely low magnification can be easily obtained by drawing from the hole diameters of the upper and lower magnetic poles of the objective lens and the position of the deflection coil. In the example of FIG. 3, a straight line connecting the inner wall of the lower deflection coil and the inner wall of the lower magnetic pole of the objective lens,
The lowest magnification can be realized if the point of intersection with the optical axis is set as the deflection fulcrum. Further, since the primary electron beam is focused on the sample by the focusing lens, the focusing angle becomes very small, and there is an effect that observation can be performed at a deep depth of focus.

[0006]

FIG. 1 is a schematic sectional view of an embodiment of the present invention. The primary electron beam 4 emitted from the cathode 1 by the voltage V1 applied to the cathode 1 and the first anode 2 is accelerated by the voltage Vacc applied to the second anode 3 and proceeds to the subsequent lens system. The primary electron beam 4 is focused as a minute spot on the sample 7 by the focusing lens 5 and the objective lens 6 controlled by the lens control power supply 15, and is scanned two-dimensionally on the sample by the two-stage deflection coils 8 and 9. You. The scanning signals of the deflection coils 8 and 9 are controlled by the deflection control circuit 14 according to the observation magnification. On the other hand, the secondary electrons 11 generated from the primary electron beam irradiation point of the sample 7 are detected by the secondary electron detector 12 and converted into an electric signal (secondary electron signal). The secondary electron signal and the scanning signal of the deflection control circuit 14 are input to the image display device 18, and an enlarged image (secondary electron image) of the sample is displayed.

The winding ratio of the deflection coils 8 and 9 is such that when the scanning signal of the deflection control circuit 14 flows through both deflection coils with the same current, the deflection fulcrum of the primary electron beam becomes the lens principal surface of the objective lens 6. It is set as follows. On the other hand, the auxiliary deflecting coil 10 is wound at the same position as the deflecting coil 9, and the deflection signal can be turned on / off by the deflecting fulcrum switch 16. Further, the objective lens 6 is also configured to be able to be turned on / off by the exciting current switch 17.

FIG. 2 shows the states of the changeover switches 16 and 17 under normal high-resolution observation conditions, and the deflection trajectory (solid line and dotted line) of the primary electron beam 4. Under normal observation conditions, since the changeover switch 16 is in the OFF state,
The auxiliary deflection coil 10 is not operating. Therefore, the primary electron beam 4 is deflected around the main surface of the objective lens as a fulcrum, as shown in FIG. The changeover switch 17 is ON
In this state, an exciting current suitable for focusing the primary electron beam on the sample flows through the objective lens.

FIG. 3 shows the states of the changeover switches 16 and 17 and the deflection trajectory (solid line and dotted line) of the primary electron beam when performing ultra-low magnification observation. When performing observation at an extremely low magnification, the changeover switch 16 is turned on, and the auxiliary deflection coil 10 is excited. Therefore, due to the combined action of the deflection coils 9 and 10, the amount of deflection of the primary electron beam by the lower deflection coil becomes larger than the normal condition (the condition in FIG. 2). As a result, the deflection fulcrum moves above the state shown in FIG. 2, so that the amount of deflection on the sample becomes larger than the normal deflection (the state shown in FIG. 2), as is clear from FIG. Can be displayed. Further, since the excitation of the objective lens 6 is OFF (the switch 17 is in the OFF state), no distortion occurs even if the deflection fulcrum is displaced from the position of the main surface of the objective lens. At this time, the primary electron beam is focused on the sample by the focusing lens 5.

FIG. 4 is a schematic sectional view showing another embodiment of the present invention. In this embodiment, the deflection coils 8 and 9 are controlled by independent driving circuits 19 and 20, respectively, instead of using the auxiliary deflection coils. The drive circuit 20
The gain can be variably set, and the deflection fulcrum can be switched between when a very low magnification condition is desired and when a normal observation condition is set. According to the configuration of FIG. 4, even if an electrostatic deflector is used instead of the deflection coil, the deflection fulcrum can be arbitrarily changed by changing the gain of the drive circuit (in this case, the voltage control circuit). Observation at extremely low magnification becomes possible.

[0011]

According to the characteristic structure of the present invention, it is possible to easily realize the observation of an extremely low magnification image which is lower than the minimum magnification obtained by ordinary deflection.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of one embodiment of the present invention.

FIG. 2 is a diagram showing a state of each deflection coil and a deflection trajectory of a primary electron beam under normal deflection conditions.

FIG. 3 is a diagram showing the state of each deflection coil and the deflection trajectory of a primary electron beam under extremely low magnification deflection conditions.

FIG. 4 is a schematic sectional view of another embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Cathode, 2 ... First anode, 3 ... Second anode, 4 ... Primary electron beam, 5 ... Focusing lens, 6 ... Objective lens, 7 ... Sample, 8 ...
Upper deflection coil, 9 Lower deflection coil, 10 Auxiliary deflection coil, 11 Secondary electrons, 12 Secondary electron detector, 13
... Objective aperture, 14 deflection control circuit, 15 lens control circuit, 16 ultra-low magnification changeover switch, 17 objective lens excitation cutoff switch, 18 image display device, 19 upper side deflector drive circuit, 20 lower side Variable drive circuit for side deflector.

Claims (5)

(57) [Claims] 1. A electron source focusing lens system for irradiating the sample with finely focused primary electron beam emitted from, you two-dimensionally scanning the primary electron beam on the sample is deflected in two stages two Step deviation
And a detector to detect electrons emitted from the sample.
In the scanning electron microscope obtained, the deflection fulcrum of the primary electron beam is moved at least near the lens main surface.
Near the electron source from the lens main surface.
At least one stage of the two-stage deflector so that
A scanning electron microscope comprising means for controlling a director. 2. A have you to claim 1, for switching the deflection fulcrum, Scanning, characterized in that is provided an auxiliary deflector for deflecting fulcrum switched to at least one of the deflector of the two-stage deflector electronic microscope. 3. A have you to claim 1, wherein the independently circuitry driving circuit for driving the respective two-stage deflector, by switching the ratio of the supply current or supply voltage, to the two-stage deflector, the two A scanning electron microscope characterized by switching a deflection fulcrum of a step deflector. 4. A scanning electron microscope according to claim 1, wherein the excitation condition of the final-stage focusing lens (objective lens) is switched in conjunction with the switching of the deflection fulcrum of the two-stage deflector. (5) Narrow the primary electron beam emitted from the electron source
A focusing lens system for irradiating the sample with the primary electron
Two-step deflection that deflects the line in two steps and scans the sample two-dimensionally
And a detector to detect electrons emitted from the sample.
In the scanning electron microscope obtained, The deflection fulcrum of the primary electron beam is defined by the optical axis of the primary electron beam.
At least two points The two-stage deflector to move
Control means for controlling the objective lens of the focusing lens system.
At least in the first excitation condition or the first excitation condition.
The second excitation to make the state weaker or non-energized than the magnetic condition
Setting means for setting the condition.
The deflection fulcrum is located closer to the electron source than the other deflection fulcrum.
When attached to the objective lens under the second excitation condition
A scanning electron microscope characterized by exciting a laser beam.