JPS5821063Y2 - Jikainosayouchikeshisouchi - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a device for canceling the effect of an external magnetic field in an apparatus of the type that uses an electron beam, such as a scanning electron microscope.

External magnetic fields include direct current magnetic fields, but the most nuisance is the commercial frequency alternating magnetic fields created by currents flowing through indoor power lines and electrical equipment, and this invention aims to cancel the effects of this field. It is an alternating magnetic field.

Normally, in electron microscopes, etc., a method of surrounding it with a magnetic material is used to shield external magnetic fields, but if you want to make some object protrude from inside or make it possible to see inside optically, a magnetic shielding material is used. Because a window hole must be drilled, the magnetic field penetrates inside.

The present invention attempts to eliminate the effect of external magnetic fields in a largely open structure without structurally sealing the space in which the effects of external magnetic fields are to be removed, as described above. It is possible to freely move structural objects in and out of the space, enter and exit the optical system light path, etc.

The present invention does not directly shield the external magnetic field, but cancels the influence of the external magnetic field by compensating for the deflection of the electron beam due to the external stray magnetic field by controlling the magnetic field using an electron beam deflection coil.

The present invention will be explained below with reference to Examples.

The purpose and principle of the present invention will be explained in further detail.

Consider what the stray magnetic field caused by an external magnetic field inside an electron microscope looks like.

The external magnetic field is generated by commercial alternating current transformers and power distribution lines, and if we take a relatively small area around the center of a fairly large room, we can assume that it has an approximately uniform or gradual spatial distribution within that area. It may be possible to cancel such a magnetic field by measuring the magnetic field in or near the area and using the output as a control signal.

The outer casing of an electron microscope is made of iron, so it is shielded from most external magnetic fields, but the magnetic permeability of iron is not infinite, and the outer casing is divided into sections with non-magnetic material sandwiched between them. Therefore, when you look at the outer casing as a magnetic shield, there are gaps here and there.

Therefore, each part of the outer casing is magnetized by an external magnetic field, and the magnetic flux generated by this magnetization leaks into the casing through these gaps, and the stray magnetic field within the electron microscope has large local changes.

Moreover, the external shapes of the outer casings are not the same, and the flow of eddy currents is not the same.The temporal changes in leakage magnetic flux in the gaps between the divided outer casings are due to the effects of the magnetic hysteresis of the material and the eddy currents. Each gap has a different waveform from the external magnetic field.

The purpose of the present invention is to eliminate the disturbance of electron trajectories caused by such temporally and spatially disparate magnetic fields.

In such a case, if an external stray magnetic field is detected somewhere in the electron microscope and used directly as an S control signal, it is impossible to cancel out the effect of the temporally and spatially disparate magnetic field as described above.

The present invention has solved the above-mentioned problems by considering the following.

The stray magnetic field in the space that electrons pass through varies greatly spatially, and the waveform and phase of the temporal changes in each part of the space are not the same.

However, the cause of such a magnetic field is almost the same as that of a stray magnetic field at a suitable location outside the device.

Therefore, it can be seen that the stray magnetic field in the electron orbital space is made up of the same components as the external magnetic field.

Therefore, it can be seen that the final result of orbital disturbance when electrons fly under the influence of such a stray magnetic field consists of the same component as the external magnetic field.

Therefore, instead of canceling the stray magnetic field in each location individually in the electron orbit space, it is sufficient to use a single magnetic field generator to compensate for the final result of the orbit disturbance, and such a magnetic field can be generated from the above-mentioned locations. Since the cause is obviously the external stray magnetic field, it should be possible to form it by suitably scaling and phase shifting the components of the external stray magnetic field.

FIG. 1 is a block diagram showing the overall configuration of an apparatus according to an embodiment of the present invention.

X and Y are compensation deflection coils that give compensation deflections in the X and y directions to the electron beam, and xs and ys are coils that detect external magnetic fields in the X and y directions, respectively.

Strictly speaking, the external magnetic field should be detected in the space where the electron beam exists, but since spatial changes in the external magnetic field are relatively gradual, it is not necessary to provide Xs and ys close to the electron beam. There is no problem since it is far away from the deflection coils X and Y, and its position is quite flexible.

When an external magnetic field penetrates the detection coils xs and ys, X S
An induced electromotive force is induced at +ys, and this voltage is amplified by a gain-adjustable preamplifier A, and then filtered by a filter circuit Fx,
After being decomposed into component waves in Fy, each component wave is passed through phase shifters Sx1, Sx2, Sx3 and Syl, Sy2 .
Mixing circuit MX9My after the phase shift is adjusted by Sy3
, each component wave is summed up again, the power is amplified, and the deflection coil
, swept away by Y.

The filter circuits Fx and Fy both include a plurality of Bandhus filters f1. f2. f3, where fl passes the commercial frequency, fl passes twice the frequency, and f3 passes the frequency three times the commercial frequency.

Since the external alternating magnetic field is significantly distorted even though it is caused by commercial alternating current, a simple sine wave at the commercial power frequency is insufficient for the compensation deflection output, and it is necessary to add a distorted wave component.

Moreover, since the detection coil and the compensation deflection coil are in different positions, the phase relationship of the component waves is also slightly shifted due to external influences, so they are decomposed into component waves and phase shifted.

It undergoes a process called mixing.

In this case, not only the phase relationship but also the ratio of each harmonic to the fundamental wave has changed somewhat, so when adding in the mixing circuit, the first. The coefficient multiplied by the second harmonic is also adjustable.

Adjustment of the above device is performed as follows.

Regarding the case of a scanning electron microscope, while looking at the image of the standard sample, first check the gain of the preamplifier and the phase shifter Sx1.
．． Adjust Syl to correct rough image distortion.

Since the scanning speed of the electron beam is matched to the commercial frequency,
The image does not waver even under the influence of an external magnetic field (it is only statically distorted).

Once the large distortion is removed, next is SX2. Adjust Sy2, then adjust Sx3 and Sy3 to gradually eliminate fine distortions.

Figure 2 shows filter Fx, phase shifter Sxl, Sx2°Sx
3. This is a circuit diagram showing a part of the mixing circuit Mx, where y of the external magnetic field
The circuit for the direction component has exactly the same structure.

Filters f1, by amplifiers AI, A3, A5
fl, f3 are configured, and amplifiers A2 . A4 and A6 constitute phase shifters Sx1, Sx2, and Sx3.

The mixing circuit Mx is an adder circuit constituted by an amplifier AI, and the first . The coefficient multiplied by the second harmonic component is adjusted.

FIG. 3 is a simplified diagram of another embodiment, showing a circuit for -harmonic components.

xs, ys are external magnetic field detection coils in the X direction and y direction,
, Y is a compensation deflection coil, and Ax s Ay is a part including a preamplifier, a filter, and a phase shifter for the first harmonic in FIG.

Ax' and Ay' are output amplifiers, each of which is an adder circuit, and both take an appropriate amount of the output of Ax vAy and mix it.

With this configuration, it is not necessary to match the directions of the axes of the coils X and xs and Y and ys.

In the above two embodiments, the compensation deflection coils X and Y are separate from the electron beam scanning deflection coil.
It goes without saying that both of these signals may be used, and the scanning signal and the compensation signal may be added and sent through one coil.

The present invention has the above-mentioned configuration, and instead of shielding or erasing the external magnetic field itself, the unnecessary deflection of the electron beam caused by the external magnetic field is canceled out by the compensation output of the deflection coil. The area around the electron beam is magnetically sealed, but the area around the electron beam is open, and various devices and objects such as optical microscopes and X-ray spectrometers are placed around the electron beam, and windows are provided in the device body. etc., the degree of freedom is significantly expanded.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of an apparatus according to an embodiment of the present invention, FIG. 2 is a circuit diagram of a part of the above, and FIG. 3 is a circuit diagram of another embodiment of the present invention. X, Y... Deflection coil in the X direction and the y direction, x
s, ys... Coils that detect external magnetic fields in the X and Y directions.

Claims (1)

[Scope of utility model registration request] It has a coil that deflects the electron beam and a coil that detects a stray magnetic field.The output of this detection coil is decomposed into a fundamental wave and harmonics, and the phase of each of the above parts is adjustable by a phase shifter. , an external magnetic field in an electron microscope in which the amplitude of each wave is appropriately adjusted and then the summed output controls the current flowing through the deflection coil, and the deflection of the electron beam due to a stray magnetic field is compensated by the deflection coil. device to cancel the effect of