JPH07161327A - Focusing method in charged particle beam - Google Patents

Abstract

PURPOSE:To realize a focusing method in a charge particle beam capable of focusing easily in a short time and in high accuracy. CONSTITUTION:An exciting electric power 16 supplies exciting current varying in step shaped synchronized with a vertical scan signal to an objective lens 3. Accordingly, a scanning area of an electron beam of a sample is irradiated by an electron beam in a different focusing condition by every horizontal scanning line. A secondary electron signal detected by a detector 6 in a focusing condition due to the respective step shaped exciting current is converted into a digital signal by an AD converter 12 and then supplied to a memory 13 and stored in it. An integrating signal stored in the memory 13 is compared by a CPU 14 and a line for obtaining a max. integrating signal is found out and exciting current of this line is supplied from an exciting electric power 16 to a focusing lens 3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a focusing method in a charged particle beam apparatus such as a scanning electron microscope capable of automatically focusing a charged particle beam.

[0002]

2. Description of the Related Art A scanning electron microscope has an automatic focusing function. In this focusing, the excitation of the focusing lens is changed stepwise, and a predetermined region of the sample is scanned with the electron beam in each excitation state, that is, in each focusing state of the electron beam. Electrons and backscattered electrons are detected, and detection signals are integrated for each focusing state. Then, the integrated values of the detection signals in each focusing state are compared, the focusing state when the maximum value is obtained is determined to be the focus position, and the excitation of the focusing lens is set in that state.

[0003]

As another method of focusing, a continuous sweep signal synchronized with the vertical scanning signal is given to the focusing lens. In this case, since the focusing state of the electron beam changes as the sample is vertically scanned,
When observing the screen, the focused state can be confirmed at a specific horizontal scanning position. An offset current is superimposed on the focusing lens so that this focused state is located at the center of the screen. Finally, focusing is completed by applying the exciting condition of the focusing lens in the central portion of the screen (the central portion of the vertical scanning signal) over the entire scanning area.

In the above-mentioned focusing method, the operation for bringing the central portion of the screen into the focused state is performed manually, and there is a drawback that the scanning is complicated and lacks in accuracy, and further it takes time.

The present invention has been made in view of the above circumstances, and an object of the present invention is to realize a focusing method for a charged particle beam capable of performing focusing with high accuracy in a short time in a simple manner. is there.

[0006]

A focusing method in a charged particle beam apparatus according to the present invention scans a focusing lens for focusing a charged particle beam on a sample and an irradiation position of the charged particle beam on the sample. Particle beam apparatus including a scanning means for scanning, a detector for detecting a signal obtained by irradiating the sample with the charged particle beam, and a means for continuously changing the focused state of the charged particle beam on the sample In, the focusing state of the charged particle beam is continuously changed in synchronization with the vertical scanning signal, the detection signals in each focusing state of the charged particle beam are integrated with respect to the signal detected by the detector, and each integrated signal Is stored, the optimum focus position is obtained from the stored series of integrated values, and the focusing lens is set at the optimum focus position.

Further, regarding the signal detected by the detector, the focused state of the charged particle beam is continuously changed, and the focused state of the charged particle beam is changed in synchronization with the vertical scanning signal at each change. It is characterized in that the detection signals in each focusing state of the charged particle beam for each unit change amount are integrated, the optimum focus position is obtained based on each integrated signal, and the focusing lens is set at the optimum focus position.

[0008]

The focusing method in the charged particle beam apparatus according to the present invention continuously changes the focused state of the charged particle beam in synchronization with the vertical scanning signal, and integrates the detection signals in each focused state of the charged particle beam. , Each integrated signal is stored, the optimum focus position is obtained from the stored series of integrated values, and the focusing lens is set at the optimum focus position.

[0009]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 shows an example of a scanning electron microscope for carrying out a focusing method according to the present invention, and 1 is an electron gun. Electron beam EB generated from electron gun 1
Is finely focused on the sample 4 by the focusing lens 2 and the objective lens 3. Further, the electron beam EB is applied to the deflection coil 5
And the electron beam irradiation position on the sample 4 is scanned. Secondary electrons generated by the irradiation of the sample 4 with the electron beam are detected by the secondary electron detector 6. The detection signal of the detector 6 is amplified by the amplifier 7 and then supplied to the cathode ray tube 9.

The detection signal is supplied to the memory 13 via the AD converter 12. The AD converter 12 or the memory 13 is controlled by the CPU 14. 15
Is an operation panel, and the operation panel 15 sends an instruction signal to the CPU 14. The CPU 14 is a scanning signal generation circuit 1 that supplies a scanning signal to the excitation power supply 16 of the objective lens 3 and the deflection coil 5.
Control 7 The operation of such a configuration is as follows.

When observing a normal secondary electron image, the CPU 14 controls the scanning signal generating circuit 17 to a normal scanning state based on an instruction signal from the operation panel 15. A predetermined scanning signal is supplied from the scanning signal generation circuit 17 to the deflection coil 5, and an arbitrary region on the sample 4 is scanned by the electron beam EB. Secondary electrons generated by irradiating the sample 4 with the electron beam are detected by the detector 6. The detection signal is supplied to the cathode ray tube 9 synchronized with the scanning signal to the deflection coil 5 through the amplifier 7, and the secondary electron image of an arbitrary region of the sample is displayed on the cathode ray tube 9.

Next, the case of performing the focusing operation of the electron beam will be described. When the operation panel 15 is operated and the focusing mode is instructed, the CPU 14 causes the objective lens 3
Excitation power supply 16 and scanning signal generation circuit 17 for deflection coil 5
And control. By this control, the excitation power supply 16 supplies the objective lens 3 with an excitation current that changes stepwise in synchronization with the vertical scanning signal as shown in FIG. During this time, the scanning signal generation circuit 17 supplies the scanning signal for operating the predetermined region of the sample to the deflection coil 5. As a result, the electron beam scanning region of the sample is irradiated with the electron beam in a different focusing state for each horizontal scanning line. FIG. 2B shows the screen of the cathode ray tube 9, but this scanning screen is horizontally scanned in 512 steps (lines) in the vertical direction, and the focusing state of the electron beam changes for each horizontal scanning line. is doing.

The secondary electron signal detected by the detector 6 in the focus state by the stepwise exciting current is amplified by the amplifier 7 and then AD converter 12
After being converted into a digital signal by the memory 13
Are stored and stored in.

By the steps described above, the secondary electron detection signal is stored in the memory 13 for each horizontal scanning line. Then, the step of scanning the predetermined region of the sample while changing the excitation state of the focusing lens in the vertical direction is executed many times, during which the secondary electron detection signal is stored in the memory having the function of the recursive filter processing. Thirteen
Integral processing is performed inside. As a result, the SN ratio of the stored secondary electron detection signal is significantly improved.

When the above-mentioned predetermined number of integration processes are completed,
The CPU 14 performs an integration process of the secondary electron signals for each 512 lines stored in the memory 13, compares the obtained integration signals for each line, and finds a line at which the maximum integration signal is obtained. FIG. 2C shows the signal value stored in the memory 13, where the vertical axis represents the vertical scanning position and the horizontal axis represents the integrated signal intensity. In the case of FIG. 2C, the maximum integrated signal is obtained on the line L _k . CPU13
Finds an exciting current supplied from the exciting power source to the focusing lens at the line L _k , and controls the exciting current to be supplied from the exciting power source to the focusing lens. By performing two-dimensional scanning of the electron beam in the excited state of the focusing lens, it is possible to observe the scanning electron microscope image in a focused state.

Now, in the above-mentioned system, the horizontal scanning line L
_If there is no unevenness on the sample surface above _{k and} it is a smooth surface,
You cannot get the expected peak value. For example, in FIG.
In the case where the uneven portion Q exists only near the non-focused horizontal scanning line L _{n in} (b), in the above method, the integrated value of the signal based on the horizontal scanning line L _n is Not necessarily big because it doesn't exist.

In the second embodiment of the present invention for solving such a problem, an exciting current as shown in FIG. 3 is supplied from the exciting power supply 16 to the objective lens 3. That is, when the exciting current synchronized with the vertical scanning signal as shown in FIG. 3A is supplied in the first sample scanning period, it is shown in FIG. 3B in the second sample scanning period. Figure 3 like
An exciting current in which the offset current I _o is superimposed on the step-like exciting current in (a) is supplied.

Next, in the third sample scanning period, as shown in FIG.
A step-like exciting current in which the offset current 2I _o is superimposed on the step-like exciting current of FIG. 3A as shown in FIG. 3C is supplied. In this way, the offset current I _o ~I _n is superimposed on the excitation current i changes stepwise in synchronization with the vertical scanning signal, a sample scanning n times is performed.

By such an operation, for example, I _k +
If there is a focus at the k step line, (k + 1) step line, and (k + 2) step line of the screen at i, I _{k + 1} + i, and I _{k + 2} + i, the lines at which the respective focus points are sequentially located at the center of the observation screen. The DC current signal is changed as described above. The respective DC currents at that time are averaged, and the averaged current value is set as the focused current value. By carrying out such a treatment, even if a part of the scanning region of the sample has uneven portions, and most of the sample surface is smooth, it is possible to perform accurate focusing. In addition, in this embodiment, since the focused current values of a plurality of times are averaged, the SN ratio can also be improved.

Although one embodiment of the present invention has been described in detail above, the present invention is not limited to this embodiment. For example, although secondary electrons are detected, reflected electrons may be detected. Further, in the embodiments, the scanning electron microscope has been described as an example, but the present invention can also be applied to an apparatus using an ion beam. Further, although the excitation of the objective lens was changed during the focusing operation, an auxiliary lens of the objective lens may be provided and the excitation of the auxiliary lens may be changed. Furthermore, the focusing state of the electron particle beam may be continuously changed, and may be changed stepwise as in the above-mentioned embodiment or may be changed linearly.

[0021]

As described above, the focusing method in the charged particle beam apparatus according to the present invention continuously changes the focused state of the charged particle beam in synchronization with the vertical scanning signal, so that each of the charged particle beam can be changed. Since the detection signals in the focused state are integrated, each integrated signal is stored, the optimum focus position is obtained from the stored series of integrated values, and the focusing lens is set at the optimum focus position, so it is easy and short time. To
Focusing can be performed with high accuracy.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a scanning electron microscope for carrying out the method of the present invention.

FIG. 2 is a diagram for explaining the operation of the embodiment of the present invention.

FIG. 3 is a diagram showing an exciting current to an objective lens in another example of the present invention.

[Explanation of symbols]

 1 Electron Gun 2 Focusing Lens 3 Objective Lens 4 Sample 5 Deflection Coil 6 Detector 7 Amplifier 9 Cathode Ray Tube 12 AD Converter 13 Memory

Claims (2)

[Claims] 1. A focusing lens for focusing a charged particle beam on a sample, a scanning means for scanning an irradiation position of the charged particle beam on the sample, and a sample obtained by irradiating the sample with the charged particle beam. In a charged particle beam apparatus equipped with a detector for detecting a signal and means for continuously changing the focused state of a charged particle beam on a sample, the focused state of the charged particle beam is continuously synchronized with a vertical scanning signal. , The detection signal in each focusing state of the charged particle beam is integrated with respect to the signal detected by the detector, each integrated signal is stored, and the optimum focus position is obtained from the stored series of integrated values. A focusing method in a charged particle beam device in which a focusing lens is set at a focal position. 2. A focusing lens for focusing a charged particle beam on a sample, a scanning means for scanning an irradiation position of the charged particle beam on the sample, and a sample obtained by irradiating the sample with the charged particle beam. In a charged particle beam apparatus comprising a detector for detecting a signal and means for continuously changing the focused state of a charged particle beam on a sample, the focused state of the charged particle beam is continuously changed and each change In each case, the focused state of the charged particle beam is changed in synchronization with the vertical scanning signal, and with respect to the signal detected by the detector, the detection signals in each focused state of the charged particle beam are integrated,
A focusing method in a charged particle beam apparatus, wherein an optimum focus position is obtained based on each integrated signal, and a focusing lens is set at the optimum focus position.