JPH05307942A - Emission noise cancelling method for scanning type electron microscope - Google Patents

Abstract

PURPOSE:To remove noise owing to emission current fluctuation from an electron gun on a picture by providing an offset adjusting circuit, an integrator, and a computing processing control means setting offset amount to the offset adjusting circuit. CONSTITUTION:A CPU 14 monitors integrated value obtained from an integrator 12 as varying offset amount in a scanning stop state, obtains such offset amount that the integrated value becomes minimum, and sets the offset amount to an offset adjusting circuit 8. That is, an offset correction mode is provided, and the variation amount absolute value of the divided results of a noise cancelling signal obtained by detecting a portion of electron beam and a signal emitted from a sample 5, as varying gradually the offset amount in the sample scanning stop state. The integrated value is monitored, the offset amount is set so that the integrated value becomes minimum, and the set up offset amount is deducted from a detected signal. Thereby it is possible to remove surely emission noise owing to emission current fluctuation from an electron gun on a picture, removing offset always.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an emission noise canceling method for a scanning electron microscope which is designed to eliminate the influence of fluctuations in the intensity of electrons emitted from a field emission electron gun.

[0002]

2. Description of the Related Art In general, electrons emitted from a field emission electron gun contain a fluctuation of several percent. This is because the work function of the metal surface changes due to the adsorption of gas on the emitter surface or the migration of the adsorbed gas and ions, and the shape of the metal surface changes due to the collision of ions and the like. Therefore, when the field emission electron gun is used for a scanning electron microscope, a noise canceling detector is provided in the middle of the lens barrel to detect electrons in the vicinity of the probe, and this detection signal is emitted from the sample. Emission noise on an image is eliminated by dividing the signal that is generated.

FIG. 4 (a) illustrates such a conventional noise canceling method, in which a part of the electron beam 21 emitted from the field emission electron gun 20 is detected by a noise canceling diaphragm 22. The electron beam that has passed through the objective diaphragm 23 is scanned by the scanning coil 24 and emitted from the sample 25.
The secondary electron is detected by the detector 26, and the detected signal Z and the signal X detected by the noise canceling diaphragm 22 are divided by a divider 27.
By dividing by Z / X to eliminate the fluctuation included in the electron beam, the CRT 28 displays the sample image for observation.

[0004]

By the way, when an offset amount is generated between the noise canceling signal and the signal from the sample, an error occurs in the division result, so the offset amount must be removed in advance. For example, in FIG.
As shown in (b), when a sample in a furnace 31 having a heater is irradiated with an electron beam through the objective lens 30 and a signal is detected by the detector 32, the detector 32 generates heat in addition to the secondary electrons. Electrons are detected at the same time, and these are superposed on the secondary electron signal. Further, when a PN junction is used for the backscattered electron detector 33, the optical component generated from the furnace is superposed on the backscattered electron signal and detected, and the If the signal superposed by the optical component is V ₀ , the output of the divider becomes (Z + V ₀ ) / X, and it becomes impossible to remove the influence of the fluctuation of the emission current intensity. Therefore, for example, an offset circuit 34 is connected to the backscattered electron detector to manually set a constant offset value to cancel the V ₀ , but the offset amount changes due to temperature fluctuations in the furnace or the like. Therefore, V ₀ cannot be completely removed,
As a result, it was difficult to eliminate the emission noise from the image.

The present invention has been made to solve the above problems. Even if noise such as an optical signal or thermoelectrons is mixed in the detector, the offset amount due to these noises is removed, and the fluctuation of the emission current from the electron gun on the image is eliminated. It is an object of the present invention to provide a noise canceling method for a scanning electron microscope capable of removing noise due to noise.

[0006]

The present invention detects a part of an electron beam emitted from a field emission electron gun, and a detection signal and a signal obtained by detecting electrons emitted from a sample. In a scanning electron microscope that cancels emission noise by dividing by a divider, an offset adjustment circuit provided between a detector that detects electrons emitted from a sample and a divider, An integrator that integrates the amount of change in the output signal of the divider and an arithmetic processing control unit that inputs the integrator output and sets an offset amount to the offset adjustment circuit are provided, and the arithmetic processing control unit stops scanning. In this state, the integrated value obtained from the integrator is monitored while changing the offset amount, and the offset amount that minimizes the integrated value is obtained and set in the offset adjustment circuit.

[0007]

According to the present invention, the scanning electron microscope using the field emission electron gun is provided with the offset correction mode, and a part of the electron beam is detected while changing the offset amount stepwise while the sample scan is stopped. The absolute value of the change amount of the division result of the obtained noise canceling signal and the signal emitted from the sample is integrated, the integrated value is monitored, the offset amount that minimizes this value is set, and it is set from the detection signal. By subtracting the offset amount, it is possible to always remove the offset and reliably remove the emission noise due to the fluctuation of the emission current from the electron gun on the image.

[0008]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing a relationship between an offset amount and an integrated value, and FIG. 3 is a diagram explaining a processing flow. In the figure, 1 is a field emission electron gun,
2 is a diaphragm for noise cancellation, 3 is a scanning coil, 4 is a sample chamber, 5 is a sample, 6 is a detector, 7 is a scanning power supply, 8 is an offset adjusting circuit, 9 is a divider, 10 is a CRT, 11 is a high pass. Filter, 12 is integrator, 13 is A / D converter,
Reference numeral 14 is a CPU, and 15 and 16 are D / A converters.

Electrons emitted from the field emission electron gun 1 are applied to the sample 5 through the noise canceling diaphragm 2 and the scanning coil 3. Secondary electrons emitted from the sample 5 are detected by the detector 6, and the detection signal Z is input to the divider 9 through the offset adjusting circuit 8. At the same time, the noise canceling signal X detected by the noise canceling diaphragm 2 is input to the divider 9, Z / X is calculated, and the changed amount is input to the integrator 12 through the high-pass filter 11. The integrator 12 integrates the absolute value of the change in Z / X, and the integrated value is passed through the A / D converter 13 to the CPU 14
Is taken into. The CPU 14 stops the scanning of the scanning power supply 7 so that the intensity of the electron beam emitted from the sample 5 becomes constant, and the offset value set in the offset adjusting circuit 8 is changed, so that the offset is adjusted as shown in FIG. The offset value Voffest that minimizes the integrated value is obtained by monitoring the integrated value with respect to the amount. At this time, since Voffest that minimizes the integrated value is equal to the offset amount included in the detection signal, this value is set in the offset adjustment circuit.

As shown in FIG. 3, when the offset correction mode is first selected, the CPU calculates the offset amount by the CPU.
Reference numeral 14 outputs a constant control signal to the scanning power source 7 to stop scanning and stop sample scanning (step 1
00, 101). Next, the offset V is set to the initial value V _0.
, And the noise cancellation signal at that time and the secondary electron signal from the sample are detected (steps 102 and 10).
3). The division result of these signals is passed through a high-pass filter, for example, for one screen scan on the cathode ray tube or for a certain period of time to obtain an integrated value I, and the integrated value I and offset value V are stored as Im and Voffset, respectively (step 106). Next, the offset value is increased by ΔV (step 108), the noise cancellation signal and the signal from the sample are detected again, and the division results are integrated for one screen.
The integrated value I at that time is compared with the stored integrated value Im, and if the integrated value I is smaller, Im is updated with the value I and the offset value at that time is Voffset.
Also, the offset amount that minimizes the integrated value can be obtained by updating the above, and then performing the same processing for all offset values.

The offset amount thus obtained is set in the offset adjustment circuit 8 and subtracted from the signal detected by the detector 6, whereby the offset can be completely removed. As a result, the noise cancellation signal is used to detect the detection signal from the sample. Emission noise can be canceled by dividing. When the calculated offset amount is used as a fixed value, it may be stored in a RAM or the like.

By the above processing, it is possible to obtain a sample image in which the emission noise is always erased on the image.

[0013]

As described above, according to the present invention, the scanning electron microscope using the field emission electron gun is provided with the offset correction mode, and by using the offset mode, the secondary electron detected by the heating stage or the like is used. Even if an optical signal from the sample is mixed in the reflected electron signal, the offset amount of these signals can be removed, and the noise due to the fluctuation of the emission current from the electron gun on the image can be precisely removed.

[Brief description of drawings]

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

FIG. 2 is a diagram showing a relationship between an offset amount and an integrated value.

FIG. 3 is a diagram illustrating a processing flow.

FIG. 4 is a diagram illustrating noise cancellation of a conventional electron microscope.

[Explanation of symbols]

1 ... Field emission electron gun, 2 ... Noise canceling diaphragm,
3 ... Scanning coil, 4 ... Sample chamber, 5 ... Sample, 6 ... Detector,
7 ... Scanning power source, 8 ... Offset adjusting circuit, 9 ... Divider,
10 ... CRT, 11 ... High-pass filter, 12 ... Integrator, 13 ... A / D converter, 14 ... CPU, 15,16 ...
D / A converter.

Claims (1)

[Claims] 1. A part of an electron beam emitted from a field emission electron gun is detected, and a signal obtained by detecting this detection signal and an electron emitted from a sample is divided by a divider. In a scanning electron microscope that cancels emission noise by means of an offset adjustment circuit provided between the detector that detects the electrons emitted from the sample and the divider, and the amount of change in the divider output signal. And an arithmetic processing control means for inputting an integrator output and setting an offset amount to the offset adjusting circuit, and the arithmetic processing control means changes the offset amount in the scanning stopped state. Emission noise of a scanning electron microscope characterized in that the integrated value obtained from the integrator is monitored and the offset amount that minimizes the integrated value is set and set in the offset adjustment circuit. Cancellation method.