JPH08273576A - Focusing method in electron beam apparatus and electron beam apparatus - Google Patents

Abstract

(57) [Summary] [Object] To realize a focusing method and an electron beam apparatus in an electron beam apparatus capable of preventing saturation of an image signal system during automatic focusing and performing a correct focusing operation. [Structure] When the operation panel 15 is operated to instruct a focusing mode, an operation instruction signal is supplied from the operation panel 15 to the contrast / brightness controller 20.
The controller 20 controls the contrast adjuster 18 and the brightness adjuster 19 based on this instruction signal. First, under the control of the controller 20, the voltage supplied from the brightness adjuster 19 to the differential amplifier 7 is set to 0V. Further, in the controller 20, the supplied signal and two kinds of threshold values L1 and L
2 and controls the contrast adjuster 18 so that the signal is between this threshold. In this way, the signal is adjusted appropriately for automatic focusing.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a focusing method and an electron beam apparatus in an electron beam apparatus such as a scanning electron microscope, which is optimum for automatically focusing an electron 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.

FIG. 1 shows an example of such a scanning electron microscope, and 1 is an electron gun. The electron beam EB generated from the electron gun 1 is finely focused on the sample 4 by the focusing lens 2 and the objective lens 3 (final stage focusing lens). Further, the electron beam EB is deflected by the deflection coil 5, and the irradiation position of the electron beam 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 high-pass filter 8 and the cathode ray tube 9.

The signal passed through the high pass filter 8 is supplied to the integrator 11 via the absolute value circuit 10. Integrator 1
The integration result of 1 is stored in the memory 13 via the AD converter 12.
Is supplied to. The signal stored in the memory 13 is appropriately supplied to the control circuit 14. Reference numeral 15 is an operation panel, and the operation panel 15 sends an instruction signal to the control circuit 14. Control circuit 14
Controls the driving power supply 16 for the objective lens 3 and the deflection control circuit 17 for the deflection coil 5.

A photomultiplier tube is used as the secondary electron detector 6, and a photoelectron multiplication voltage is applied from the contrast adjuster 18 to the detector 6, and the contrast is adjusted by this voltage. . A differential amplifier is used as the amplifier 7 that amplifies the secondary electron detection signal, and the voltage from the brightness adjuster 19 is supplied to the negative terminal of the amplifier 7. As a result, the offset amount of the image signal from the detector 6 is optimized, and the brightness is adjusted. The operation of such a configuration is as follows.

When observing a normal secondary electron image, the control circuit 14 controls the deflection control circuit 17 based on an instruction signal from the operation panel 15, and the deflection control circuit 17 sends a predetermined scanning signal to the deflection coil 5. It is supplied, and an arbitrary two-dimensional area on the sample 4 is raster-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.

At this time, the contrast of the image, that is, the magnitude of the amplitude of the image signal can be adjusted by controlling the high voltage supplied from the contrast adjuster 18 to the secondary electron detector 6. Also, the brightness of the image is
By changing the voltage supplied from the brightness adjuster 19 to the negative terminal of the differential amplifier 7, the offset amount of the signal is changed, whereby the optimum state is adjusted.

Next, a case where a normal electron beam focusing operation is performed will be described. When the operation panel 15 is operated to instruct the focusing mode, the control circuit 14 controls the driving power supply 16 for the objective lens 3 and the deflection control circuit 17 for the deflection coil 5. As a result, the driving power supply 16 supplies the objective lens 3 with a stepwise exciting current, and the deflection control circuit 17 sends a scanning signal for scanning a predetermined region of the sample each time the stepwise exciting current changes. Deflection coil 5
Supply to.

The secondary electron signal detected by the detector 6 in the focus state by each step-like exciting current is amplified by the amplifier 7, the direct current component is removed by the high-pass filter 8, and then the absolute value circuit. It is converted by 10 into a positive signal. The output of the absolute value circuit 10 is supplied to the integrator 11, and the signal based on one scanning of the electron beam for each excitation step of the objective lens 3 is integrated. The integrated value of the integrator 11 is converted into a digital signal by the AD converter 12, and then supplied to the memory 13.

The memory 13 stores the integrated value of the integrator 11 for each excitation step of the objective lens 3.
FIG. 2 shows the change in the stored integral value at this time.
The vertical axis represents the integrated value, and the horizontal axis represents the excitation intensity of the objective lens 3.
The control circuit 14 smooths the change curve of the integrated value stored in the memory 13 and then performs fitting with a curve made up of a quadratic function to excite the objective lens 3 at the peak of the change in the integrated value of the signal amount. Detect intensity. Control circuit 14
Controls the driving power supply 16 to set the objective lens 3 to the excitation intensity at the peak, and the focusing operation is performed in this way.

[0011]

By the way, in the above focusing, the image signal system at the time of automatic focusing is also used as a circuit for observing an image by the cathode ray tube. Therefore, if the contrast or brightness of the image signal is improperly adjusted, the signal system will be saturated and the output of the high-pass filter 8 will be zero. As a result, the integrator 11
The integration result of is also zero, and the maximum value cannot be detected.

Such a phenomenon occurs when the accelerating voltage of the electron beam EB is changed, when the observation visual field is changed, or when the constituent elements of the electron beam scanning region of the sample are different.

The present invention has been made in view of the above circumstances, and an object thereof is to prevent an image signal system from being saturated during automatic focusing and to perform an accurate focusing operation. A method of focusing in an apparatus and an electron beam apparatus are realized.

[0014]

According to a first aspect of the present invention, there is provided a focusing method in an electron beam apparatus, wherein the intensity of a focusing lens for focusing an electron beam on a sample is changed stepwise, and each time the change is made, the intensity of the focusing lens is changed. , Scanning a predetermined region on the sample with an electron beam, integrating the signal obtained with this scanning, based on the integrated value for each stepwise intensity of the focusing lens, to detect the intensity of the optimum focusing lens, In the focusing method in the electron beam apparatus in which the focusing lens intensity is set to the intensity, the intensity level of the signal obtained by scanning is adjusted to fall within a predetermined range, and then the focusing operation is performed. The feature is that it is executed.

In the electron beam apparatus according to the invention of claim 2, a focusing lens for focusing the electron beam on the sample and a means for changing the intensity of the focusing lens for focusing the electron beam on the sample in a stepwise manner. A scanning means for scanning a predetermined region on the sample with an electron beam, a detector for detecting a signal obtained by irradiating the sample with the electron beam, and an adjustment for adjusting the intensity level of the signal from the detector. A detector, a means for comparing the intensity level of the signal from the detector with the two predetermined levels, and a controller for controlling the intensity level of the signal from the detector to fall within the two predetermined levels; The integrated means for integrating the adjusted signal from the and the integrated signal for each step-like intensity change of the focusing lens are compared, and the intensity of the focusing lens is set to the intensity when the maximum integrated intensity is obtained. It is characterized in that a means for.

[0016]

According to the focusing method of the electron beam apparatus according to the present invention, the intensity level of the signal obtained by the scanning of the electron beam on the sample is adjusted so as to fall within a predetermined range, and then the focus is adjusted. Perform a matching operation.

In the electron beam apparatus according to the second aspect of the present invention, in order to perform automatic focusing, the adjustment for adjusting the intensity level of the signal from the detector for detecting the signal obtained by irradiating the sample with the electron beam. And a means for comparing the intensity level of the signal from the detector with the two predetermined levels and controlling the regulator so that the intensity level of the signal from the detector falls within the two predetermined levels. .

[0018]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 3 shows a scanning electron microscope which is an embodiment of the present invention. The same or similar components as those of the conventional apparatus of FIG. 1 are designated by the same reference numerals, and detailed description thereof will be omitted. In this embodiment, a contrast / brightness controller 20 in the automatic focusing mode is provided.
The signal detected by the secondary electron detector 6 and amplified by the amplifier 7 is supplied to the controller 20 via the AD converter 21. The controller 20 also controls the contrast adjuster 17
And the brightness adjuster 18 are controlled. The operation of such a configuration will be described below.

When observing a normal secondary electron image, the control circuit 14 controls the deflection control circuit 17 based on the instruction signal from the operation panel 15, and the deflection control circuit 17 controls the same as in the conventional apparatus shown in FIG. A predetermined scanning signal is supplied to the deflection coil 5, and an arbitrary two-dimensional area on the sample 4 is raster-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 sent to the deflection coil 5 via the amplifier 7.
Is supplied to the cathode ray tube 9 synchronized with the scanning signal to the cathode ray tube 9, and the secondary electron image of an arbitrary region of the sample is displayed on the cathode ray tube 9.
At this time, the contrast of the image is adjusted by the contrast adjuster 18
And the brightness of the image is adjusted by the brightness controller 19.

Next, a case where a normal electron beam focusing operation is performed will be described. When the operation panel 15 is operated to instruct the focusing mode, the control circuit 14 controls the driving power supply 16 for the objective lens 3 and the deflection control circuit 17 for the deflection coil 5. As a result, the driving power supply 16 supplies the objective lens 3 with a stepwise exciting current, and the deflection control circuit 17 sends a scanning signal for scanning a predetermined region of the sample each time the stepwise exciting current changes. Deflection coil 5
Supply to.

At this time, an instruction signal for the focusing operation is supplied from the operation panel 15 to the contrast / brightness controller 20, and the controller 20 controls the contrast adjuster 18 and the brightness adjuster 19 based on this instruction signal.
First, the brightness adjuster 1 is controlled by the controller 20.
The voltage supplied from 9 to the differential amplifier 7 is 0V.
As a result, the output of the amplifier 7 will not be saturated unless the output of the secondary electron detector 6 is saturated.

The signal from the amplifier 7 is supplied to the AD converter 2
After being AD-converted by 1, it is supplied to the contrast / brightness controller 20. The controller 20 compares the supplied signal with two types of threshold values L1 and L2. For example, if the signal S is between two types of threshold values L1 and L2 as shown in FIG. 4A, the focusing operation similar to that of the conventional apparatus is performed with normal signal strength.

That is, the secondary electron signal detected by the detector 6 in the focus state by the stepwise excitation current passes through the amplifier 7, the direct current component is removed by the high pass filter 8, and then the absolute value circuit. It is converted by 10 into a positive signal. The output of the absolute value circuit 10 is supplied to the integrator 11, and the signal based on one scanning of the electron beam for each excitation step of the objective lens 3 is integrated. The integrated value of the integrator 11 is converted into a digital signal by the AD converter 12, and then supplied to the memory 13.

The memory 13 stores the integrated value of the integrator 11 for each excitation step of the objective lens 3, and the control circuit 14 controls the objective lens at the peak of the change curve of the integrated value stored in the memory 13. The excitation intensity of 3 is detected. The control circuit 14 controls the driving power supply 16 and sets the objective lens 3 to the excitation intensity at the peak.

Next, the contrast from the AD converter 21
The intensity of the signal S supplied to the brightness controller 20 is
If the threshold value L1 is exceeded as shown in FIG.
The controller 20 controls the contrast adjuster 18 and reduces the voltage output supplied from the adjuster 18 to the secondary electron detector 6. As a result, the intensity level of the signal obtained from the detector 6 becomes low, but this signal is again contrasted,
It is supplied to the brightness controller 20 and both thresholds L1 and L
Compare with 2. By repeating such an operation, the focusing operation is performed when the signal from the detector 6 falls between the threshold values.

Further, the contrast from the AD converter 21
The intensity of the signal S supplied to the brightness controller 20 is
As shown in FIG. 4C, when the threshold value is L2 or less, the controller 20 controls the contrast adjuster 18 to increase the voltage output supplied from the adjuster 18 to the secondary electron detector 6. As a result, the intensity level of the signal obtained from the detector 6 increases, but this signal is supplied again to the contrast / brightness controller 20 and compared with both threshold values L1 and L2. By repeating such operations, the focusing operation is performed when the signal from the detector 6 falls between the two threshold values.

In this way, the detection signal level from the detector 6 is monitored, and the operation for adjusting the detection signal level to an appropriate level for automatic focusing is performed prior to the focusing operation. Does not saturate and can properly perform signal integration. It should be noted that the comparison between the detection signal and the threshold value described above is made on the screen 1
It is performed by averaging the signal sample values for the frames, or using the maximum and minimum values of the signal.

When the focusing operation is completed, the operation panel 1
When the image observation mode is designated by 5, the controller 20 controls the contrast adjuster 18 and the brightness adjuster 19, and the voltage signals before the automatic focusing operation are respectively supplied to the secondary adjuster including the photomultiplier tube from each of the adjusters. It supplies to the electronic detector 6 and the differential amplifier 7.

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, the voltage applied to the secondary electron detector is changed in order to adjust the intensity level of the image signal during the focusing operation, but the present invention is not limited to this configuration, and for example, the amplification factor of the signal amplification system may be changed. You can

[0030]

As described above, according to the present invention, the intensity level of the signal obtained by scanning the electron beam on the sample is adjusted so as to be within a predetermined range, and then the focusing operation is executed. I decided to do it. As a result, when the accelerating voltage of the electron beam EB is changed, when the observation field of view is changed, or when the constituent elements of the scanning region of the electron beam of the sample are different, the signal system is saturated and the maximum value is detected. The phenomenon that makes it impossible is prevented.

[Brief description of drawings]

FIG. 1 is a diagram showing a conventional scanning electron microscope having an automatic focusing function.

FIG. 2 is a diagram showing a relationship between an excitation intensity of an objective lens and an integrated value.

FIG. 3 is a diagram showing an embodiment of a scanning electron microscope according to the present invention.

FIG. 4 is a diagram showing two types of threshold values and signal levels.

[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 14 control circuit 15 operation panel 16 drive power supply 17 drive power supply 18 contrast adjuster 19 brightness adjuster 20 contrast and brightness Controller 21 AD converter

Claims (2)

[Claims] 1. The intensity of a focusing lens for focusing an electron beam on a sample is changed stepwise, and a predetermined region on the sample is scanned by the electron beam each time the intensity is changed. Focusing method in an electron beam device in which the optimum focusing lens strength is detected based on the integrated value for each stepped strength of the focusing lens and the focusing lens strength is set to that strength. 2. A focusing method in an electron beam apparatus, comprising: adjusting the intensity level of a signal obtained by scanning within a predetermined range, and then performing the focusing operation. 2. A focusing lens for focusing the electron beam on the sample, a means for changing the intensity of the focusing lens for focusing the electron beam on the sample stepwise, and an electron beam for a predetermined region on the sample. Scanning means for scanning, a detector for detecting the signal obtained by irradiating the sample with the electron beam, a controller for adjusting the intensity level of the signal from the detector, and an intensity of the signal from the detector Means for comparing the level with two predetermined levels, controlling the regulator so that the intensity level of the signal from the detector falls within the two predetermined levels, and integrating for integrating the regulated signal from the detector Means for comparing the signals integrated for each step-like intensity change of the focusing lens, and means for setting the intensity of the focusing lens to the intensity when the maximum integrated intensity is obtained. .