JP3032397B2 - Field emission 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 a field emission type scanning electron microscope which corrects the influence of fluctuations in detection signal intensity based on changes in the amount of emission electron current of a field emission electron gun.

[0002]

2. Description of the Related Art FIG. 1 shows an example of a conventional field emission type scanning electron microscope, and 1 is a field emission type electron gun. The field emission type electron gun 1 includes a field emission emitter 2, an extraction electrode 3, and an anode 4. A predetermined extraction voltage is applied to the extraction electrode 3, and an acceleration voltage is applied between the emitter 2 and the anode 4. By applying, the electron beam EB
Is obtained.

The accelerated electron beam EB generated from the electron gun 1 is focused on a sample 7 by a focusing lens 5 and an objective lens 6. Although not shown, a deflection coil for deflecting the electron beam along the optical axis is provided, and by supplying a scanning signal to the deflection coil, the electron beam E
B scans the sample two-dimensionally. A beam fluctuation detector 8 is provided below the electron gun 1 so as to cut a part of the electron beam generated from the electron gun.

[0004] Secondary electrons generated from the sample based on the irradiation of the sample 7 with the electron beam EB are detected by a secondary electron detector 9. The detection signal of the detector 9 is supplied to the divider 11 via the amplifier 10. The current value detected by the beam fluctuation detector 8 is supplied to the divider 12 by the amplifier 12.
Is supplied via The output signal of the divider 11 is supplied to a cathode ray tube 13 to which a scanning signal from a scanning signal generator (not shown) is supplied. The operation of such a configuration will now be described.

[0005] The electron beam EB from the electron gun 1 is narrowly focused on a sample 7 by a focusing lens 5 and an objective lens 6. At this time, a scanning signal from a scanning signal generator is supplied to the deflection coil, and the electron beam scans a predetermined area of the sample 7. 2 generated by irradiation of the sample 7 with the electron beam
The next electron is detected by the detector 9, and the detection signal is amplified by the amplifier 10 and then supplied to the divider 11.

Now, it is known that the current value of the electron beam from the field emission type electron gun 1 fluctuates with time.
When the amount of current of the electron beam applied to the sample 7 fluctuates,
Inevitably, the amount of secondary electrons generated from the sample also fluctuates. For this reason, the beam fluctuation detector 8 monitors the current amount of the electron beam generated from the electron gun 1, and guides a signal corresponding to the current amount of the electron beam and a secondary electron detection signal to the divider 11, and outputs the signal to the divider 11. 11 divides both signals. As a result, the detection signal is normalized, and a signal from which noise due to the fluctuation of the current amount of the irradiation electron beam is canceled is obtained from the divider 11. Accordingly, by supplying the output signal of the divider 11 to the cathode ray tube 13, an image which is not affected by the fluctuation of the current of the electron beam from the electron gun 1 is obtained on the cathode ray tube 13.

[0007]

By the way, in the field emission scanning electron microscope, the accelerating voltage of the electron beam is 0.1%.
A wide range of voltages of ３０30 kV is used. The accelerating voltage is
Although the voltage is changed by changing the voltage between the emitter 2 and the anode 4, the voltage applied to the extraction electrode 3 is usually constant. Therefore, when the acceleration voltage is changed, the voltage between the extraction electrode 3 and the anode 4 is changed. Will greatly change. An electrostatic lens is formed by the extraction electrode 3 and the anode 4, and the lens effect changes due to a change in acceleration voltage, that is, a voltage difference between the extraction electrode 3 and the anode 4.

For this reason, the opening angle of the electron beam extracted from the anode differs according to the change in the lens action, and the amount of the electron beam detected by the beam fluctuation detector 8 greatly differs depending on the acceleration voltage. Particularly in extreme cases, the electron beam may not be detected at all by the beam fluctuation detector 8. If the electron beam is not detected by the beam fluctuation detector 8 at all, or if the signal amount is detected, the normalization of the secondary electron detection signal by the divider 11 cannot be performed properly.

The present invention has been made in view of such a point, and an object thereof is to always appropriately correct a secondary electron detection signal even when an acceleration voltage in a field emission type electron gun is changed. The object of the present invention is to realize a field emission scanning electron microscope capable of performing the above.

[0010]

A field emission scanning electron microscope according to the present invention comprises a field emission emitter, an extraction electrode for extracting electrons from the emitter, an anode for accelerating the extracted electrons, and an accelerating electrode. A focusing lens for narrowly focusing the electron beam on the sample, a scanning unit for two-dimensionally scanning the electron beam on the sample, and a detection for detecting a signal generated by irradiating the sample with the electron beam. Device, shielding means for cutting a part of the electron beam at two places along the optical axis of the electron beam optical system, and two types of amplifiers for amplifying the current amount of the electron beam incident on each shielding means, respectively. And adding means for adding the outputs of the two amplifiers, dividing means for dividing the output of the detector and the output of the adding means, and image display means to which the output of the dividing means is supplied. Ri is characterized by being configured to vary according to the amplification degree of the two amplifiers to changes in acceleration voltage.

[0011]

The field emission scanning electron microscope according to the present invention
A first and a second beam fluctuation detector are provided along the optical axis of the electron beam, and a signal obtained by adding the outputs of the two types of detectors is divided by a secondary electron detection signal and the like to obtain a sample. The influence of the fluctuation of the secondary electron detection signal due to the fluctuation of the current amount of the irradiated electron beam is corrected.

[0012]

An embodiment of the present invention will be described below in detail with reference to the drawings. FIG. 2 shows a scanning electron microscope according to the present invention. The same parts as those of the conventional apparatus of FIG. In this embodiment, the stop of the objective lens 6 is set to the second beam fluctuation detector 15.
The output signal of the second beam fluctuation detector 15 is supplied to an adder 17 via a variable amplifier 16.

The output signal of the first beam fluctuation detector 8 is supplied to an adder 17 via a variable amplifier 18. The adder 17 includes two types of beam fluctuation detectors 16 and 18.
And outputs the added signal to the divider 11. 1
Reference numeral 9 denotes a control circuit. The control circuit 19 is supplied with a signal corresponding to the acceleration voltage from an acceleration voltage generation circuit (not shown) of the electron gun 1. The control circuit 19 includes two types of variable amplifiers 16 and 18. To control the amplification factor. The operation of such a configuration will now be described.

The electron beam generated from the electron gun 1 and accelerated is focused by a focusing lens 5 and an objective lens 6 and irradiates a sample 7, and a part thereof is cut by a first beam fluctuation detector 8. And further cut by the second beam fluctuation detector 15. The signals detected by the first and second beam fluctuation detectors 8 and 15 are amplified by variable amplifiers 16 and 18, respectively, supplied to an adder 17, and added. This addition signal is proportional to the amount of current of the electron beam applied to the sample 7, and the addition signal and the secondary electron detection signal from the detector 9 are divided by the divider 1
As a result, a signal is obtained in which the noise due to the fluctuation of the current amount of the electron beam from the electron gun 1 is canceled as in the conventional apparatus. The output signal of the divider 11 is supplied to a cathode ray tube 13, so that a secondary electron image is displayed on the cathode ray tube.

Here, the acceleration voltage of the electron gun 1 is changed,
When the acceleration voltage is increased, the electrostatic lens action by the extraction electrode 3 and the anode 4 becomes stronger, and the opening angle of the electron beam from the electron gun 1 becomes smaller, so that the electrons detected by the first beam fluctuation detector 8 The proportion of the beam decreases, and instead, the proportion of the electron beam detected by the second beam fluctuation detector 15 increases.

On the other hand, when the accelerating voltage of the electron gun 1 is changed and the accelerating voltage is lowered, the electrostatic lens action between the extraction electrode 3 and the anode 4 is weakened, and the opening angle of the electron beam from the electron gun 1 is increased. Therefore, the first beam fluctuation detector 8
, The ratio of the electron beam detected by the second beam fluctuation detector 15 decreases.

As described above, as the acceleration voltage of the electron gun 1 is changed, the amount of current detected by each beam fluctuation detector changes, but the change in the amount of current of the electron beam of the secondary electron detection signal changes. Since the addition signal of the two types of beam fluctuation detectors 8 and 15 is used for correction, even if the signal amount from one of the detectors becomes extremely small or is not detected,
The secondary electron detection signal can be appropriately corrected. Note that the amplification factors of the variable amplifiers that amplify the detection signals of the first and second beam fluctuation detectors 8 and 15 are individually changed in accordance with the acceleration voltage. However, the output of the adder 17 having a substantially constant SN ratio can be obtained.

Although the embodiment of the present invention has been described in detail, the present invention is not limited to this embodiment. For example, secondary electrons are detected, but the present invention can also be applied to the case where reflected electrons are detected and a reflected electron image is displayed. Although the objective lens aperture is used as the second beam variation detector 15, a second beam variation detector may be separately provided above the objective lens 6 in addition to the objective lens aperture. However, since the objective lens aperture is smaller than the diameter of the first beam variation detector 8 and can detect an electron beam nearer on the axis, it is desirable to use the objective lens aperture as the second beam variation detector. Furthermore, in the above embodiment, two beam fluctuation detectors are provided and their outputs are added. However, two or more beam fluctuation detectors may be provided and their outputs may be added.

[0019]

As described above, the field emission type scanning electron microscope according to the present invention is provided with the first and second beam fluctuation detectors along the optical axis of the electron beam. It is configured to divide the signal obtained by adding the output of the detector and the secondary electron detection signal, etc., and to correct the influence of the fluctuation of the secondary electron detection signal due to the fluctuation of the current amount of the electron beam irradiated on the sample. Therefore, even when the acceleration voltage in the field emission electron gun is changed, the secondary electron detection signal can always be appropriately corrected.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a conventional field emission scanning electron microscope.

FIG. 2 is a diagram showing one embodiment of a field emission scanning electron microscope according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Field emission type electron gun 2 Emitter 3 Extraction electrode 4 Anode 5 Focusing lens 6 Objective lens 7 Sample 8 First beam fluctuation detector 9 Secondary electron detector 10 Amplifier 11 Divider 13 Cathode ray tube 15 Second beam fluctuation detection Unit 16, 18 Variable amplifier 17 Adder 19 Control circuit

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int. Cl. ⁷ , DB name) H01J 37/04 H01J 37/073 H01J 37/28 H01J 37/09

Claims (1)

(57) [Claims] 1. A field emission emitter, an extraction electrode for extracting electrons from the emitter, an anode for accelerating the extracted electrons, and a focusing lens for finely focusing the accelerated electron beam on a sample. Scanning means for two-dimensionally scanning an electron beam on a sample, a detector for detecting a signal generated by irradiating the sample with the electron beam, and two points along an optical axis of an electron beam optical system. A shielding means for cutting a part of the electron beam, two kinds of amplifiers for amplifying the current amount of the electron beam incident on each shielding means, an adding means for adding outputs of the two kinds of amplifiers, Divider means for dividing the output of the adder and the output of the adder means, and image display means to which the output of the divider means is supplied. The amplification degree of the two kinds of amplifiers changes according to the change of the acceleration voltage. Let Field emission scanning electron microscope configured as follows.