JP2000077018A - Focusing device for scanning electron microscope - Google Patents

Abstract

(57) Abstract: The present invention relates to a focusing device for a scanning electron microscope, and an object of the present invention is to provide a focusing device for a scanning electron microscope that can accurately perform axis alignment of an objective aperture. SOLUTION: A wobble signal generator for periodically changing a voltage of an acceleration power supply for alignment of an objective diaphragm,
Comparison means for comparing the specified magnification stored at each accelerating voltage with the actually set magnification, and controlling the current of the objective lens when the set magnification is smaller than the specified magnification as a result of the comparison. And a means for controlling the voltage of the acceleration power supply when the set magnification is equal to or larger than the specified magnification as a result of the comparison.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a focusing mechanism of a scanning electron microscope, and more particularly, to a focusing method when a wobble is applied to a high-voltage power supply and an objective aperture is aligned.

[0002]

2. Description of the Related Art A column of a scanning electron microscope (SEM) comprises an electron source for generating an electron beam (electron beam), a scanning system for scanning the electron beam, and a lens system for using the electron beam as a thin probe. I have. Further, an objective aperture is provided for limiting the opening angle of the electron beam irradiating the sample. A wobble function is provided in the objective lens power supply to align the axis of the objective aperture.

[0003] This wobble function, as shown in FIG.
The current flowing through the coil is periodically changed to monitor the movement of the observation image generated at this time, and the position of the objective aperture is adjusted so that the movement is minimized. In FIG. 2, the vertical axis represents the objective lens current, and the horizontal axis represents time. When the wobble is turned on, a minute AC having a period of about 0.5 seconds is superimposed on the objective lens current.

However, when observing a high-resolution image, it may be better to adjust the objective aperture by applying a wobble to a high-voltage power supply for accelerating an electron beam. The reason for this is that in high-voltage wobble, not the objective lens alone, but the overall axis including the focusing lens that constitutes the lens barrel is aligned. It is considered that the spread of electrons focused on the sample can be suppressed to a small level even if the spread exists.

FIG. 3 is a diagram for explaining the alignment of the objective aperture. When the electron beam passes outside the optical axis L on the main surface of the objective lens,
The electron beam focused on the intersection of the sample 2 with the optical axis L of the objective lens as shown at f1 in (a) is applied to a position shifted from the optical axis of the objective lens of the sample 2 by defocusing as shown at f2. . For this reason, the image appears to shift when a wobble is applied.

Here, when the axis of the objective aperture is adjusted,
(B) The electron beam focused on the intersection with the objective lens optical axis of the sample 2 as indicated by f1 in the region centered on the objective lens optical axis of the sample 2 even when defocusing is performed as indicated by f2. Irradiation. For this reason, the image does not seem to shift even if a wobble is applied.

[0007]

In the high-pressure wobble method described above, when the objective lens is adjusted and focused, an image shift occurs, which is difficult to use and has not been put to practical use.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has as its object to provide a focusing mechanism of a scanning electron microscope capable of accurately aligning an objective aperture.

[0009]

According to the present invention, there is provided a wobble signal generator for periodically changing a voltage of an accelerating power supply for positioning an objective aperture, and a memory for storing a wobble signal at each accelerating voltage. Comparing means for comparing the designated magnification with the actually set magnification, and means for controlling the current of the objective lens if the set magnification is equal to or less than the designated magnification, and a result of the comparison. And a means for controlling the voltage of the acceleration power supply when the set magnification is equal to or larger than the specified magnification.

According to the configuration of the present invention, instead of the conventional wobble for periodically changing the current of the objective lens for adjusting the position of the stop of the objective lens, the wobble system for periodically changing the acceleration voltage is changed. Accordingly, when observing an image at a specified magnification or higher at each acceleration voltage, the fine adjustment of the focus is switched from the control of the objective lens current to the fine adjustment of the acceleration voltage. Thereby, the focusing of the scanning electron microscope can be accurately performed.

[0011]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of the present invention. In the figure, 1 is an objective lens for forming an electron beam on a sample 2, 2 is a sample,
Reference numeral 3 denotes an objective stop for limiting the opening angle of the electron beam. Reference numeral 4 denotes a converging lens for converging an electron beam, and reference numeral 5 denotes an electron gun for generating an electron beam.

Reference numeral 6 denotes a detector for detecting reflected electrons or secondary electrons from the sample 2, reference numeral 7 denotes a wobble signal generator for applying a low-frequency AC signal to an acceleration power source, and reference numeral 8 denotes an output of the wobble signal generator 7. Alternatively, it is a control system that receives a control fine adjustment signal from the objective lens system and outputs a superimposed signal to the acceleration power supply. Reference numeral 9 denotes an acceleration power supply for superimposing a signal from the control system 8 on a generated voltage.

Reference numeral 11 denotes an objective lens power supply for driving the objective lens 1, and reference numeral 12 denotes a control system for providing a control signal for controlling the objective lens power supply 11. Reference numeral 10 denotes a focus signal generation circuit that generates a focus signal, and 13 denotes a switch that receives an output of the focus signal generation circuit 10 and switches between an a contact and a b contact. When the magnification is less than the designated magnification, the contact a is selected and enters the control system 12. When the magnification is greater than the designated magnification, the contact b is selected and enters the control system 8. The operation of the system configured as described above will be described below.

Conventionally, when the electron beam is focused on the sample, the excitation of the objective lens is adjusted. For this adjustment, a knob or the like is used, but the amount of change is adjusted according to the observation magnification. That is, the amount of change in one step of the knob is set large at low magnifications and small at high magnifications.

According to the present invention, when observing an image at a magnification close to the limit of resolution at each acceleration voltage, the high voltage is finely adjusted for focusing. First, a wobble signal is generated from the wobble signal generator 7 and superimposed on the acceleration power supply 9. At this time, the amplitude of the acceleration voltage is about several tens eV. The amplitude of this voltage is set according to the acceleration voltage and the observation magnification. Then, fine-adjust the position of the objective aperture 3,
Adjust so that the movement of the image is minimized.

On the other hand, the magnified image of the sample 2 is scanned with the focused electron beam on the sample, and the signal detected by the detector 6 is displayed on a display device (not shown) such as a CRT and observed. For this focusing, the currently set magnification is compared with the specified magnification at each acceleration voltage.

When the currently set magnification is equal to or less than the designated magnification, a control signal is supplied to the control system 12 to control the objective lens power supply 11. On the other hand, when the magnification exceeds the specified magnification,
When focusing is performed by the objective lens power supply 11, an image shift occurs. This is because the objective stop 3 is adjusted by a high-pressure wobble, and the axis of the objective wobble is not aligned.

Therefore, when the magnification is equal to or higher than the specified magnification, the acceleration voltage is finely adjusted to adjust the focus. For this reason, the switch 13 selects the b contact side. Therefore, the focus signal is supplied from the acceleration power supply 9 to the electron gun 5 via the control system 8. Thereby, even if the focus is changed, an image can be observed without causing an image shift.

In the present invention, manual focusing has been described as a focusing method, but automatic focusing can also be used.
In the automatic focusing, conventionally, the excitation of the objective lens is changed stepwise, the video signal obtained from the sample is differentiated, the absolute value is integrated, and the excitation of the objective lens is maximized so that the integrated value becomes maximum. I am adjusting.

In this case as well, if the excitation of the objective lens 1 is changed when the objective diaphragm 3 is adjusted by a high-pressure wobble, a slight image shift occurs. In this regard, a wider variable range of the focus is better. Therefore, focus adjustment is performed by the objective lens 1.

In the above-described embodiment, the description has been made by limiting the wobble to the accelerating voltage. However, the present invention is not limited to this, and is used in combination with the conventionally used method of wobbling the objective lens power supply. There is also a way to do this.

In this case, a method is conceivable in which the objective aperture position adjusted by each of the wobbles of the objective lens power supply 11 is stored in a memory, and the objective aperture is switched by driving a motor according to the application. Further, a method is also conceivable in which the position of the objective aperture is adjusted by an electromagnetic alignment coil and the respective values are stored in a memory.

In the above-described embodiment, the case where the acceleration voltage is finely adjusted and the focus is adjusted when the magnification is equal to or more than the specified magnification is taken as an example. When the control range is exceeded, the excess is fed back to the objective lens control system, and the acceleration power supply is reset so as to enter the controllable range again.

[0024]

As described above in detail, according to the present invention, a wobble signal generator for periodically changing the voltage of an acceleration power supply for alignment of an objective aperture, and storage at each acceleration voltage. Comparing means for comparing the specified magnification with the actually set magnification, and means for controlling the current of the objective lens when the set magnification is equal to or less than the specified magnification, as a result of the comparison. As a result, when the set magnification is equal to or greater than the specified magnification, the control means controls the voltage of the acceleration power supply. Instead of wobbling, the acceleration voltage is changed periodically. When observing an image at a specified magnification or more at each acceleration voltage, fine adjustment of the focus can be performed from the control of the objective lens current to the acceleration voltage. It can be switched to the fine adjustment, it is possible to provide axial alignment mechanism of a scanning electron microscope can be performed accurately align the objective aperture.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is an explanatory diagram of a wobble.

FIG. 3 is an explanatory diagram of axis alignment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Objective lens 2 Sample 3 Objective aperture 4 Focusing lens 5 Electron gun 6 Detector 7 Wobble signal generator 8 Control system 9 Acceleration power supply 10 Focus signal generation circuit 11 Objective lens power supply 12 Control system 13 Switching device

Continuation of the front page (72) Inventor Mitsuhide Matsushita 3-1-2, Musashino, Akishima-shi, Tokyo Japan Electronic Company F-term (reference) 5C033 MM01 MM05

Claims (1)

[Claims] 1. A wobble signal generator for periodically changing a voltage of an acceleration power supply for positioning an objective aperture, a designated magnification stored at each acceleration voltage, and a magnification actually set. Means for comparing; means for controlling the current of the objective lens if the set magnification is less than a specified magnification as a result of the comparison; acceleration if the set magnification is greater than or equal to the specified magnification as a result of comparison A focusing device for a scanning electron microscope, comprising: means for controlling a voltage of a power supply.